Animal divisions table. Biological systematics
Principles of animal taxonomy. Systematics, or taxonomy, is the science of classifying organisms. The term "systematics" comes from the Latin word systema, and taxonomy comes from the combination of the Greek words "taxis" - order and "nomos" - law. Zoological classification is the distribution of animals into subordinate groups based on the study of their similarities and differences and the identification of family relationships. The main goal of taxonomy is to build a system of animals that would provide the maximum amount of information about any group of animals and would have great predictive value.
The history of taxonomy can be divided into three periods.
The first period, pre-Linnaean, was associated with the study of local faunas, their description and naming of all animals in the language in which they were described, and the creation of classifications based on certain individual characteristics. The first classification of animals, based on the degree of their perfection (gradation), was given by Aristotle (see the section “Main stages in the development of paleontology”), his system existed for almost 2000 years.
The second period is associated with the name of C. Linnaeus (1707-1778) - the founder of systematics as a science. In the tenth edition of the book “System of Nature” (1758), C. Linnaeus proposed a taxonomy based on the subordination of taxonomic units, or taxa: kingdom, class, order, genus, species and variety. He distributed all groups of animals known at that time among subordinate taxa and gave clear and precise diagnoses. For a hundred years before the publication of Charles Darwin's work "The Origin of Species" (1859), the main attention was paid to a clear limitation of species based on the recognition of their constancy. A typological concept of the species was developed, according to which each studied specimen was compared with the type specimen, and the status of the species was determined based on the degree of morphological similarity or difference. There was a rapid increase in the number of described species, both modern and extinct, and basic methods and principles of classification were developed. The proposed hierarchy of taxonomic units has not undergone any significant changes over a period of more than 200 years, except for the addition of intermediate taxa indicated below.
At the suggestion of K. Linnaeus, each species received a double Latinized name - generic and specific. For example, Homo sapiens L., Canis familiar is Carlo, Pinus silvestris L. The first word refers to the name of the genus, which usually unites several species and is the main one; the second - to the name of the species, which is, as a rule, defining, emphasizing any characteristics of the organism. In this case: Homo - man, sapiens - wise; Cams - dog, familiaris - domestic; Pinus - pine, silvestris - forest. The species name is always followed by the surname of the author (or authors) who first described this species (L. is the accepted abbreviation of the surname of C. Linnaeus). The double name of each species served as the basis for the creation of binary nomenclature, and January 1, 1758 was taken as the date from which the law of priority applies. According to this law, the first name given to any species or genus is subject to change.
With Linnaeus, a period of rapid increase in the number of described modern and extinct species began, the development of basic principles and methods of classification, and the introduction of uniform Latinized names for all studied animals and plants. The main attention was directed to the creation of the so-called “natural” system, which was understood as the disclosure of the “plan of creation”. Since evolutionists attach a completely different meaning to the concept of “natural system,” it is better to abandon this term.
A significant contribution to the taxonomy of animals was made by the work of Cuvier and Lamarck, who identified a number of new classes (cephalopods, gastropods) from invertebrates (“bloodless”) animals. This served as an impetus for the disbandment of unnatural groups and the emergence of new ones. Cuvier, on the basis of comparative anatomical studies, identified four independent groups of animals: chordates, mollusks, articulated and radiate, and developed the principle of subordination of primary and secondary characters.
The third period in the development of systematics began with the appearance of Charles Darwin’s book “The Origin of Species” (1859). Darwin rejected creationism and substantiated the principles of phylogenetic systematics, based on identifying real kinship by origin. The essence of the "natural" system has been clarified - natural groups exist because they descend from a common ancestor. Charles Darwin created the theoretical foundations of the natural (without quotes) or phylogenetic system. He established that in the process of phylogenesis two processes take place: branching and subsequent divergence, or divergence, of the emerging branches. The division of taxa should be based on the establishment of their branching (which indicates a common origin) and on the degree of difference. The degree of changes experienced by various groups during development is expressed in their placement into taxa of different ranks: classes, orders, families.
Darwin spoke about the need to create a phylogenetic taxonomy based on the identification of characters in three categories: a) characters that reveal actual kinship (homogeneous similarity); b) signs of rudimentary or atrophied organs that are not important in the life of animals, but are important for taxonomy; c) features of the embryonic structure of the compared forms, which are important for systematics.
The post-Darwinian period - a period of struggle for the recognition of evolution - was characterized by the creation of classifications of various groups of animals and plants. The concept of type began to be gradually replaced by the concept of populations, according to which species consist of variable populations, and even within large taxa deviations from the “type” characteristic of a given category are possible. Recognition of the variability of the species forced us to pay serious attention to the types of variability and to the assessment of variability by methods of population analysis and statistical methods. The creation of the classification posed a number of difficult questions to the researcher, for example: do the two forms studied represent one or two species, what determines their similarity - phylogenetic relatedness or only external, convergent similarity. All these questions represent the third task of systematics, which was posed by Darwin - the study and analysis of intraspecific variability and the identification of evolutionary factors; in solving these questions, systematics comes into contact with genetics, biogeography, ecology, comparative anatomy and paleontological data.
The study of any group from a systematic perspective goes through three stages, called alpha, beta and gamma systematics (Mayr, Linsley, Usinger, 1946). The first - alpha systematics - is the analytical stage, during which each group is studied at the modern scientific level and names are given; the second stage - beta systematics - is a systematic stage, including the unification of group elements into a system of taxa, and the third stage, or gamma systematics, is the final stage, theoretically generalizing all the results obtained. Modern taxonomy combines methods of analysis and synthesis.
Currently, a distinction is made between phylogenetic and artificial systematics. Phylogenetic systematics is based on elucidating the genetic relationships of related groups in time and space. All animals can be arranged into a systematic hierarchy consisting of taxa that gradually increase in rank. Phylogeny and systematics are considered inextricably linked as two sides of a single process of cognition of the actual history of the organic world; Moreover, if phylogeny studies family relationships and clarifies the commonality of individual taxa, then systematics seeks to divide the identified phylogenetic branches into separate subordinate taxa. Therefore, phylogeny and systematics cannot be identified.
Artificial taxonomies differ from phylogenetic ones in that organisms are grouped according to externally similar characteristics, and several types of artificial taxonomies are distinguished. Artificial taxonomy is necessarily resorted to when individual parts of organisms are classified, for example rhyncholites, aptychus, conodonts, skeletal elements of holothurians; they receive species and generic names and are even sometimes combined into higher taxa. For some of them, such as conodonts, it is still unknown to which group of organisms they belonged. For such groups, it is proposed to distinguish parataxa - special categories that do not obey the rules of zoological nomenclature.
There are artificial classifications for fundamental reasons, when the authors believe that in reality phylogenetic systems do not exist, but that there are special “natural” or typological taxonomies based on the study of a number of characteristics, allowing one to find similarities and differences based on statistical generalizations. They are important for bringing order to the diversity of organisms. Although these taxonomies claim to be “natural,” they are far from phylogenetic taxonomy and, as a rule, represent an artificial grouping of organisms. Many unrelated groups of organisms, as is known, under similar living conditions often acquire externally similar features and, with artificial taxonomy, often ended up in the same taxon.
Currently, the following subordination of taxonomic units is accepted:
Each group has its own characteristic features. Taxonomic units really exist and it is important to find criteria for their identification. The most general criteria, applicable to all groups, can be those recommended by V. E. Ruzhentsev as principles of systematics. These include the following criteria or principles: chronological, homology, ontogenetic, actualism, basic link, chorological.
Chronological principle - when identifying any taxonomic group and solving questions of phylogeny, it is important to have as accurate chronological data as possible about the group being studied, the position of its representatives in natural sections; Ignorance of chronology can lead to mistakes and incorrect conclusions.
The principle of homology is based on the study of homologous and analogous structures, and if homologous similarity stems from a common origin, then similarity comes from a commonality of adaptations to similar conditions. Comparison based on similar formations also leads to errors and the creation of artificial taxonomy; comparison based on homologous formations makes it possible to identify actual family relationships.
The ontogenetic principle makes it possible to trace the development of individual structures in the process of individual development, to identify their similarities and differences. Early stages of ontogeny indicate the relationship of larger taxa and can serve as the basis for their identification; later stages indicate related relationships of lower taxa (genera, species). The entire course of ontogenesis shows the direction of phylogenetic development of the entire group. To clarify the specific phylogeny of any group, one should especially carefully study the late ontogenetic stages in order to find features characteristic of the closest ancestors. For some groups, such as insects, this method is not applicable at all.
The principle of actualism is to compare extinct animals with modern ones: on this basis, attempts are made to restore the structure and adaptations of extinct forms. The principle of actualism can be applied with certain reservations.
The principle of the main link is based on identifying those changes that arise in a given group during the development process and subsequently lead to the emergence and formation of a new group, i.e., to identifying a node of divergence, a place of divergence of characteristics. At first, the differences that arise are very weakly expressed, then they intensify and become leading. Establishing the main link is one of the most difficult problems of taxonomy.
The chorological principle consists in identifying the distribution of organisms in space and their ecological characteristics and geographic variability.
Using these principles, a taxonomy and phylogeny of the group under study can be developed.
The lowest taxonomic unit is the species. The problem of identifying the species has always been a subject of debate. Before the publication of Darwin's work (1859), the main attention was paid to the constancy of species and their clear delimitation. Species were identified by the degree of their morphological similarities and differences; however, the researcher often encountered various difficulties associated with morphological differences between the sexes (sexual dimorphism) and age differences - larvae often differ sharply from adults. Particular difficulties with morphological criteria arose in the establishment of sympatric natural populations, i.e., such populations that are distributed in the same area, almost do not differ from each other in morphological characteristics, but do not interbreed; they are considered independent species and called sibling species.
It is now accepted that each species consists of a group of populations whose individuals actually or potentially interbreed with each other and are reproductively isolated from other species. A species is a reproductive community united by ecological unity; although it consists of individual individuals, it interacts with other species as a single whole, possessing genetic unity and a single genetic fund. These criteria are, of course, unsuitable for paleontological material, and therefore, when identifying species, it is necessary to take into account their habitats, qualitative isolation, all morphological characteristics, conduct their morphofunctional analysis and study changes in characteristics over time. With sufficiently large paleontological collections, species can be studied in space and time. The difficulties associated with identifying sibling species have already been discussed above.
In nature, polytypic species are quite widespread - species that consist of two or more subspecies. If species are not divided into subspecies, then they are called monotypic. A subspecies is an integral part of a species; it has its own range, which is part of the range of the entire species. Subspecies are always allopatric, that is, they do not occur together in the same territory.
Species are grouped into genera, genera into families, etc. Most researchers believe that higher taxa objectively exist in nature and correspond to groups that are qualitatively different from each other. Darwin showed this well when considering issues of divergence of species and issues of systematics. However, there is also an opinion that higher taxa are subjective and created only for convenience. This subjectivist point of view currently has no supporters.
Rules of zoological nomenclature. Nomenclature (Latin nomen - name, calare - to call) is a system of names for all taxa. The main task of nomenclature is to create universality, stability and correct unambiguous understanding of the same scientific names of animals. The rules of zoological nomenclature are approved at the International Zoological Congress and are mandatory for all taxonomists. The latest International Code of Zoological Nomenclature was approved in 1964 at the International Zoological Congress in London. The Zoological and Botanical Codes are independent. As scientific names, Latin, Greek or latinized words of any language are used, reflecting any features of the group (for example, the class Bivalvia - bivalves), its geographical (Timanites) or stratigraphic (neocomiensis) position, or a name dedicated to any person, real or mythological (for example, orlovi, in honor of academician Yu. A. Orlov or Neptunoceras - Neptune's horn).
All taxonomic units are divided into five groups: 1) species (species, subspecies); 2) generic (genus, subgenus); 3) family (tribe, subfamily, family, superfamily); 4) detachment-class (suborder, detachment, superorder, subclass, class, superclass) 5) typical (subtype, type, supertype). Each taxon has its own type, a kind of “standard” - a reference standard with which the studied forms are compared to eliminate any doubts and correct definitions; The type is the core of the taxon and the basis of its name; it is objective and unchangeable, but its boundaries or volume (except for the holotype) are subject to change. The type of any taxon cannot be replaced, with rare exceptions. For a species or subspecies, this will be the type specimen of the species, first described and called the holotype; all other specimens will be considered paratypes. In paleontological studies, one of the most complete and well-preserved specimens is usually selected as the holotype. The holotype is indicated when describing a new species and cannot be replaced. The terms “variety” and “form” do not obey the rules of nomenclature and are considered as infrasubspecific categories.
If, when establishing a new species, the holotype was not isolated, then all specimens of the type series are syntypes of equal value in nomenclature. From this series, any taxonomist can identify one of the syntypes as a lectotype. If the holotype, lectotype or syntypes are lost or destroyed, then any other specimen can be isolated as a neotype, subject to all necessary rules.
For a genus, a nominal species is selected, called the type species; for a family, the nominal genus on which the family name is based is selected. All members of a family group based on one typical genus - tribe, subfamily, family, superfamily - are written with the surname of the author who first named one of the listed taxa. For example, the family Hoplitidae was allocated in 1890 by H. Douville, and in 1952 Wright (C. Wright) divided it into three subfamilies: Cleoniceratinae Whitehouse, 1926, Hoplitinae Douville, Gastroplitinae Wright, 1952; H. Douville remained the author of the nominal subfamily.
The names of detachment-class and type groups differ in that they are not tied to a specific type, are separate words of Latin or Greek origin, and are always in the plural (for example, Primates - primates). For names from tribe to order, it is proposed to add the appropriate endings to the genus name, indicated in parentheses when listing taxa (see the table of taxonomic units above).
The law of priority provides for the recognition of only the name of the species or genus that was proposed first in time and published in compliance with all the rules provided for by the code. All subsequent names are considered synonyms of the first and are not used as independent names. If the same name is given to two different species within the same genus or to different genera, it is considered a homonymous name; a later identical name is invalid and must be discarded. For example, Noctua is an insect and Noctua is a bird, one of the names should be changed.
If the name does not comply with the rules of the code, then it is considered invalid and does not have a nomenclature statute, i.e., according to the nomenclature rules, it is a nomen nuda (or nomina nuda in the plural).
All names of taxa above species consist of one word, that is, they are uninominal; all species names, as shown above, consist of two words, i.e., binomial; all subspecies names consist of three words and are trinominal. The names of species and subspecies are written with a lowercase letter, the names of all higher taxa - with a capital letter. In a polytypic form, one of the subspecies is nominal, i.e., the bearer of the name. For example, if a species is named album, then one of the subspecies should be called album album, and the name of the second subspecies will consist of the name of the species album and some word emphasizing the characteristics of the selected subspecies.
According to the code, species and subspecies names must be grammatically consistent with the genus name. The adjective albus, meaning white, retains the ending -us if it is masculine, changes to -a if it is feminine, and to -um if it is neuter. The selection of species names and their coordination with the name of the genus often causes many difficulties associated with ignorance of the Latin language, ignorance of the gender (masculine, feminine or neuter) to which the name refers.
If during the study it is determined that the species should be assigned to another genus, then in this case the author’s surname is placed in brackets. However, this is recommended to be done only in special paleontological work. If a genus is divided into subgenera, then the subgenus containing the type species becomes the type, or nominal, subgenus and retains the genus name, and the second and subsequent ones receive new names, with the name of the subgenus placed in parentheses after the genus name. For example, the genus Hoplites is divided into two subgenera - Hoplites (Hoplites), Hoplites (Isohoplites).
Open nomenclature is used if the preservation of the material is poor and does not allow an accurate species identification. The name “open” or “free” nomenclature is due to the fact that the described forms are not protected by the law of priority and their names may be clarified or changed during subsequent research. There are many different designations used to describe or identify poorly preserved material. Here are some examples: if membership in a genus is unreliable, then a question mark is placed after the name of the genus; If the state of preservation does not allow one to reliably determine the species, then cf is given. (short for the word conformis - similar); If the described species is fairly well preserved, but differs from a closely related species by some characteristics that do not allow the specimen to be confidently attributed to this species, then the sign aff is placed between the name of the genus and the species. (short for the word affinis - related, close). If the researcher cannot accurately determine the species, then he only indicates that the described form belongs to a group of known species, and in this case, between the names of the genus and the species he puts the sign ex gr., which means ex grege - from the group (literally from the herd). For example, Nautilus ex gr. pompilius L. If the belonging of the taxa under study to a family, order, class, or phylum cannot be established, then incertae familiae, incertae ordinis, incertae classis, incertae phylum are written accordingly (incertae - unknown).
One of Plato's students made an attempt to distribute animals into groups based on their correspondence to one or another “idea” embodied in a set of characteristics. Without creating a full-fledged classification system, he introduced two important taxonomic categories into use: “species”, i.e. a collection of almost identical forms, and a “family” is a group of similar species. Nevertheless, his works were widely used by subsequent generations of taxonomists.
Early period of modern taxonomy.
Back in the 16th century. such prominent scientists as E. Wotton and K. Gesner continued to be content with the most primitive systems of living things. However, Wotton's critical attitude towards species clearly invented by ancient authors introduced a fresh stream into this area of \u200b\u200bknowledge, which influenced Gesner. In addition to numerous articles, Gesner published his classic Animal history (Historia animalium), where he distributed them alphabetically, combining related forms into groups. Each species was described quite accurately for the time, and all the material was presented with encyclopedic care. However, despite discussing many different issues, Gesner did not make comparisons between groups and did not address functional aspects at all. At the same time, he included his original observations in the text, which most of his predecessors did not do, and demonstrated how useful it is to supplement descriptions with drawings.
Ulysses Aldrovandi published 14 large volumes on animals, showing that some of their large groups can be divided into subgroups, and including data on the internal structure of organisms in the descriptions. In the 16th century P. Belon was the first to use comparative anatomy for classification. One of the outstanding biologists of the 17th century. there was D. Ray. Among his works, mostly related to botany, there were several zoological studies that contained an in-depth analysis of the functional relationships between animals. Rey clearly established the distinction between genus and species and formulated the concept of similar characters as the basis for identifying relationships between natural groups. The works of J. Buffon, published in the mid-18th century, played an important role in the development of taxonomy. His theories, for all their shortcomings, turned out to be very useful for biologists of subsequent generations. Buffon showed that many difficulties in taxonomy arise from the external similarity of animals that are distant from each other, but it is precisely this that makes it possible to identify more general patterns of natural history.
The beginning of modern taxonomy was laid System of nature (Systema Naturae) Carl Linnaeus. Its tenth edition, published in 1758, established a hierarchy of taxonomic categories such as phylum, class, order, genus and species. We still use not only the binomial nomenclature created by Linnaeus, but also many of the scientific names he introduced. Not all of the 4,000 animal species he described continue to remain in the groups in which he placed them, but the groups themselves have survived. Linnaeus indicated a natural unit - species - as the starting point of classification, but, following Ray and his other predecessors, he considered species unchanged. Only in the 19th century, after the advent of the evolutionary theories of Jean Lamarck and Charles Darwin, the concept of the historical transformation of living forms was established. This evolutionary doctrine and the discovery at about the same time of the basic laws of heredity formulated by Gregor Mendel served as the basis for the transformation of taxonomy into a real science.
New taxonomy.
The modern classification system, using many of the ideas and methods that emerged in the 19th century, goes much further, relying on constantly accumulating new information. Currently, the characteristics of not individual individuals, but entire populations of organisms are being systematized. A quantitative approach was added to the subjective qualitative study. Experts do not limit themselves to analyzing differences and similarities, but try to create a unified natural system. It has long been recognized that populations change and that changes that occur can be perpetuated through reproductive isolation. Accordingly, the main attention is paid to such problems as the “rate and direction” of changes (evolution) of organisms; speciation, i.e. the origin of species from ancestral forms; family ties between groups.
Terminology.
Since classification was carried out by hundreds of taxonomists, working both on the same and on different materials, it became necessary to establish certain rules and terminology. The largest groups (taxa) into which the animal kingdom is now divided are called phyla. Each type is divided successively into classes, orders, families, genera and species (sometimes intermediate categories are also distinguished, for example subtypes, superfamilies, etc.). As we move from the highest to the lowest hierarchical group, the degree of relatedness between animals belonging to the same taxon increases. Within the same species, all animals are very similar in characteristics and, when crossed, produce fertile offspring. The table below illustrates this classification system with several examples.
Type | Chordata | Chordata | Chordata | Chordata |
Subtype | Vertebrates | Vertebrates | Vertebrates | Vertebrates |
Class | Bony fish | Amphibians | Mammals | Mammals |
Squad | Herring | Anurans | Predatory | Primates |
Family | Salmonidae | Frogidae | Felines | Hominids |
Genus | Trout | Real frogs | Cats | People |
View | Brook trout | Leopard frog | Domestic cat | Homo sapiens |
Scientific name | Salmo trutta | Rana pipiens | Felis catus | Homo sapiens |
All four species belong to the same type and subtype, since they have an important common feature - a spine consisting of movably articulated vertebrae. Cat and man belong to the same class; their relationship is evidenced by the presence in both cases of hair and mammary glands in females. The frog and the fish belong to different classes; the fish has gills and a two-chambered heart, while the frog has lungs and a three-chambered heart. Cats, with their claws on their fingers and a pair of large cheek teeth of a cutting type, represent the order of carnivores, and humans represent the order of primates, because instead of claws, he has nails, and his thumbs are opposed to the rest. In all four examples, the scientific name of the animal is composed of two Latin words - the generic name (with a capital letter) and the specific epithet; in any part of the globe Salmo trutta, for example, means the same specific biological species.
Classification rules.
The procedure for naming animals is regulated by certain international rules. For species described after 1758, the name proposed by the author of the description is considered priority - it is this name that all others must use; all names used by Linnaeus are also given priority (if they correspond to the modern distribution of organisms by taxonomic groups). Two species cannot have the same name. When describing a new species, it is necessary to select and preserve in one form or another one or more of its “type” specimens, indicating the place where they were found. There are also rules about the languages that can be used for names, and about the grammatical structure of the latter (for example, their “Romanization” is required, although the use of Greek roots is acceptable).
Such general rules did not always exist: Linnaeus and other scientists used their own, which led to confusion. A number of countries tried to develop national codes of biological nomenclature, for example in Great Britain (Strickland Code, 1842), USA (Dall Code, 1877), France (1881) and Germany (1894). Finally, everyone realized that classification is an international problem. In 1901, the International Rules of Zoological Nomenclature (International Code) were adopted. There is an International Commission on Zoological Nomenclature, whose functions include recommending amendments and additions to the Rules, interpreting them, compiling lists of clarified names and resolving controversial issues of classification.
BASIC SIGNS OF ANIMALS
Despite significant differences between animal types, many of them share some fundamental characteristics that can be used to identify distant relationships. However, these similarities, for example, characteristics of growth and embryonic development, cannot be considered absolute. On the one hand, they may be characteristic not only of a given large group, and on the other hand, they may not be found in all of its representatives; in addition, they are expressed to varying degrees or not at all stages of development. Therefore, many zoologists do not consider them particularly significant. Nevertheless, such characters generally help to understand the origin and evolution of animal types and to develop a classification that most accurately reflects their relationships.
Symmetry.
One of the most important features of an organism is the symmetry of its structure. If a body can be divided into at least two identical or mirror-like parts, it is called symmetrical. Animals are characterized by two types of symmetry: bilateral (bilateral) and radiant (radial); neither one nor the other is found in its pure form. Sponges, cnidarians and ctenophores are radially symmetrical, i.e. their general shape is cylindrical or disc-shaped, with a central axis. More than two planes can be drawn through this axis, dividing the body into two identical or mirror parts. Animals of all other types are bilaterally symmetrical: the anterior (head) and posterior (tail) ends, as well as the lower (abdominal) and upper (dorsal) sides are clearly visible; as a result, the body can only be divided lengthwise into two mirror halves - right and left. It may seem that some types of animals (for example, echinoderms) are mistakenly classified as bilaterally symmetrical - their symmetry appears to be radial. However, it is secondary in origin: their ancestors had bilateral symmetry, which can be found in the larval stages of modern forms.
Egg crushing.
Another fundamental feature is the nature of egg fragmentation during the formation of the embryo. Despite the complexity and diversity of this process in different groups, two main types can be distinguished - radial and spiral.
The polar axis of an egg is an imaginary line running from its “north pole” (top) to its “south” (base). The radial crushing furrows run either perpendicular or parallel to this axis. As a result, a cluster of cells is formed, located radially and symmetrically relative to it (like slices in an orange).
The furrows of spiral cleavage run at a different angle to the polar axis, so the emerging daughter cells are located “obliquely” - slightly above and below the mother cell from which they were formed, and form spirals as part of the developing embryo.
With radial and spiral fragmentation, the timing of determining the future “fate” of the cells usually differs, i.e. what tissue will ultimately develop from one or another group of them. If this occurs only at a relatively late stage of development, then by dividing a four-cell embryo (for example, a starfish) into separate cells under experimental conditions, each of them can be grown into a whole individual. This development is called regulatory; it is usually associated with the radial type of crushing. Conversely, if the fate of the cells is determined very early, then the experimental division of a four-cell embryo (for example, a ring) will lead to the formation of only four of its “quarters”. This development is called mosaic; it is characteristic of spiral crushing.
Gastrulation.
The early embryo, formed as a result of cleavage, is essentially a spherical lump of cells called a blastula ( cm. EMBRYOLOGY). During further development, it becomes two-layered; more precisely, the process of gastrulation turns it into a gastrula. Gastrulation occurs differently depending on the type of blastula.
This process is especially clearly expressed in animals with a hollow blastula (for example, starfish): during the so-called. Invagination, a certain part of it is screwed inward and forms a pocket-like cavity. The pocket wall becomes the inner layer located under the original outer layer. For clarity, imagine a weakly inflated ball that you press with your finger - under it there will be two layers of rubber.
Germ layers.
The two layers of cells formed as a result of gastrulation are called germ layers: the outer one is the ectoderm, the inner one is the endoderm. Subsequently, a third layer, the mesoderm, is formed between them. It comes in two main types: mesenchymal (a loose mass of cells embedded in a gelatinous substance) and sheet-like (resembling epithelial tissue). In sponges, cnidarians and ctenophores, the mesoderm is mesenchymal, arising from ectoderm cells. In all other types of animals it is either mesenchymal or sheet-like and is formed from the endoderm.
Each germ layer gives rise to certain tissues and organs of the adult organism; Thus, in vertebrates, the central nervous system and receptors of sensory organs (for example, eyes) are derivatives of ectoderm, muscles and the circulatory system are mesoderm, and the liver, pancreas and thyroid glands are endoderm.
Two-layer (Diploblastica) and three-layer (Triploblastica) forms.
Sponges are so unique that they belong to neither one nor the other.
In cnidarians and ctenophores, during embryonic development, only the first two germ layers are usually formed - these animals are called bilayered. Representatives of all other types develop a third germ layer (mesoderm) - they are three-layered.
However, in many forms classified as bilayered, mesenchymal mesoderm develops, which was not previously considered as such, since it is not of endodermal, but of ectodermal origin. In this regard, the terms “three-layer” and “two-layer” are not entirely accurate, but nevertheless they often continue to be used according to tradition.
Protostomes (Protostomia) and deuterostomes (Deuterostomia).
The internal space in the form of a pocket, formed in the embryo during gastrulation, is the rudiment of the digestive tract, i.e. primary gut. The hole leading into it is called a blastopore. In some types, such as annelids, molluscs and arthropods, part of it forms the mouth of the adult. These animals are classified as protostomes because the blastopore is the first opening of the primitive gut. In other types, in particular echinoderms and chordates, the mouth of the adult does not develop from the blastopore, but from the second intestinal opening that appears later. They were called deuterostomes.
Body cavities.
In most animals, the body wall is separated from the digestive tract by a space filled with fluid. This body cavity is present, if not in an adult animal, then at least at one of the stages of its development. There are two main ways of its formation - inside the mesoderm by its stratification and between it or the primary gut.
The process of mesoderm stratification also occurs in one of two ways. For example, in annelids, mollusks and arthropods, a pair of small cavities form and grow in the loose mass of its cells (one on each side of the embryo), and in chordates and echinoderms, the mesoderm initially develops from pocket-like protrusions of the primary gut, already surrounding the rudiments of certain cavities.
The cavities in the mesoderm continue to increase, almost completely separating the body wall from the intestine (only connecting bridges are preserved). These cavities are lined with mesodermal cells, forming the so-called. peritoneum. The internal organs, compressing and deforming the peritoneum, do not come into contact with the liquid washing it, which fills the so-called. secondary body cavity, or whole (from the Greek koiloma - cavity). Animals with a coelom are called secondary cavities (coelomics).
In roundworms and some other forms, a fluid-filled cavity is formed as a result of the disappearance of most of the mesoderm, of which only a thin layer remains adjacent to the body wall. This body cavity, separating its wall (with mesodermal lining) from the intestine, is called primary, or pseudocoelom (“false cavity”), and the animals that have it are called primary cavity, or pseudocoelomic. “False cavity” in this case means that the pseudocoelum, unlike the “real” coelom, is not completely surrounded by mesodermal lining and the internal organs lie in the fluid that fills it.
In animals such as flatworms, the space between the body wall and the intestine is densely filled with mesodermal cells. Since there is no body cavity (except for the intestine), they are sometimes called cavityless (acelomic).
Use of fundamental features in classification.
Although the above overview omits many important details, it does give an idea of what characters are used to determine the most general relationships between large groups of animals.
It is believed, for example, that chordates and echinoderms are related in evolutionary terms to a fairly close relationship. When studying modern representatives of these two phyla, such as humans (chordates) and starfish (echinoderms), this seems completely incredible. However, there are more primitive modern forms of them (ascidians in chordates and crinoids in echinoderms) and even simpler extinct ones. If the pedigrees of both groups are traced back to fairly distant ancestors and we take into account that all these animals are characterized by bilateral symmetry, radial cleavage and regulated development with the formation of three germ layers, a secondary mouth and a coelom, then the idea of a close evolutionary relationship between them seems quite reasonable .
TYPES AND CLASSES OF ANIMALS
In modern classification systems, the animal kingdom (Animalia) is divided into two subkingdoms: parazoans (Parazoa) and true multicellular organisms (Eumetazoa, or Metazoa). There is only one type of parazoa - sponges. They do not have real tissues and organs; most of their cells are totipotent, i.e. capable of changing their form and function; in addition, many of their cells are motile.
In earlier systems, Protozoa, a group of very diverse single-celled organisms, were considered another subkingdom of animals. However, among the protozoa there are known plant-like (capable of photosynthesis), intermediate (with characteristics of both plants and animals) and animal-like, i.e. receiving organic food from external sources, forms. As a result, in the modern system of the five kingdoms of life, protozoa are no longer classified as part of the animal kingdom, but are considered a sub-kingdom of the protist kingdom (Protista).
Sponge type
(Porifera, from Latin porus - time, ferre - to carry). This type includes primitive multicellular animals that lead a sessile lifestyle, attached to solid substrates in water. About 5,000 species are known, most of them marine.
The body is radially symmetrical and, in principle, consists of a central (paragastric) cavity surrounded by a double-layer wall. Water enters this cavity through pores in the wall, and from there exits through a wide mouth - at its upper end; however, in some sponges the aperture is reduced or absent, which leads to increased water flow through the pores. Its movement is caused by the beating of flagella, which are equipped with cells lining the channels in the walls. Food, oxygen, sexual products and metabolic waste are carried by this almost external water.
The skeleton of sponges consists of millions of microscopic crystalline spicules (spines) or organic fibers; its structure serves as the main criterion for dividing a type into classes. Sponges are not true multicellular animals, because their cells are loosely connected and for the most part function independently of each other. Reproduction is both asexual - by external budding or through the formation of special internal buds (gemmules), and sexual, with the participation of eggs and sperm. Some species are dioecious, i.e. There are male and female individuals, others are hermaphrodites, i.e. One individual develops both male and female reproductive cells. Sponges have a very high ability to regenerate (restore lost body parts).
Class lime sponges
(Calcarea, from Latin calx - lime). Marine animals, usually no longer than 15 cm. One-, three- or four-rayed spicules consist of calcium carbonate. The system of channels in the body varies from simple to complex.
Class ordinary sponges
(Demospongiae, from Greek demos - people, spongos - sponge). Skeletons are very diverse; some species have no skeleton at all. Spicules are one- or four-rayed, siliceous. The skeleton consists of horny fibers with or without spicules. This class includes freshwater and marine organisms (of the latter, toilet sponges are well known).
Class glass, or six-beam, sponges
(Hexactinellida, from Greek hex - six, aktinos - ray). As the name of the class indicates, silica spicules are six-rayed. They often merge, forming a skeleton consisting, as it were, of glass threads (an example is the type of basket of Venus). Marine organisms, up to 90 cm long; live at depths up to 900 m.
Type Mesozoic
Plate type
(Placozoa, from Greek plako - plate, zoon - animal). The simplest animals whose cells form tissues. The only type of this type is Trichoplax adhaerens- was discovered in 1883 in Austria, in a seawater aquarium. In shape and movements, it resembles an amoeba, but consists of several thousand cells forming two layers - upper and lower, between which there is a cavity filled with liquid with contractile cells freely floating in it. As genetic studies show, lamellates are closest to cnidarians.
Type of cnidarians, or cnidarians
(Cnidaria, from Greek knide - to burn). Another common name for this type of animal is coelenterata. Radially symmetrical, mostly marine animals, armed with tentacles and unique stinging cells (nematocytes) with which they capture and kill prey.
The body wall consists of two layers surrounding the gastrovascular cavity: the outer (epidermis) of ectodermal origin and the inner (gastrodermis) of endodermal origin. These layers are separated by gelatinous connective tissue - mesoglea. The gastrovascular cavity serves to digest food and circulate water throughout the body.
In cnidarians, real nerve cells and a diffuse type nervous system (in the form of a network) appeared for the first time. Polymorphism is characteristic, i.e. the presence within the same species of forms that differ sharply in appearance. One typical form is a sessile polyp, attached to the substrate and similar to a cylinder, at the free end of which there is a mouth surrounded by tentacles; another form is a free-swimming jellyfish, resembling an inverted bowl or umbrella with tentacles hanging from the edges. Polyps form jellyfish by budding. They, in turn, reproduce sexually: the fertilized egg develops into a larva, which gives rise to a polyp. Thus, in the life cycle of many cnidarians there is an alternation of sexual and asexual generations. Species that do not have a medusoid form reproduce sexually or by budding. They can be dioecious or hermaphroditic.
The simple cnidarians include the hydra, which reaches 2.5–3 cm in length and leads a solitary lifestyle. Many form extensive colonies. Approximately 10,000 species have been described, grouped into three classes.
Hydroid class
(Hydrozoa, from Greek hydro - water, zoon - animal). The gastrovascular cavity is not divided by radial septa. Mesoglea does not contain cells. The life cycle may include both a polyp and a jellyfish, or only one of these forms. In jellyfish, along the bottom edge of the umbrella there is a fold directed inward - the velum. A widespread freshwater form is hydra ( Hydra). In the open sea, brightly colored colonies with a “float” - the so-called - are often found. Portuguese man-of-war, whose tentacles reach a length of 12 m.
Class scyphoid
(Scyphozoa, from Greek skyphos - bowl, zoon - animal). Scyphoids include the so-called. scyphojellyfish that live exclusively in seawater. These are dioecious animals without a pronounced polyp stage in the life cycle. There is no velum, but the mesoglea contains cells. Eared jellyfish are often found ( Aurelia), reaching a diameter of more than 2 m.
Class coral polyps
(Anthozoa, from Greek anthos - flower, zoon - animal). Exclusively sessile polyps without a medusa stage in the life cycle. They live in shallow waters, most in warm seas. The gastrovascular cavity has incomplete radial septa, and the mesoglea is connective tissue. This class includes reef-forming corals, sea feathers, sea anemones and other forms. Individual individuals are almost microscopically small, but colonies consisting of them can form huge limestone structures and even islands. The diameter of some large sea anemones exceeds 30 cm. Approx. 6000 types of class.
Type ctenophores
(Ctenophora, from the Greek kteis, ktenos - comb, phoros - bearing). Mostly planktonic animals living in warm seas. The transparent bodies are biradially symmetrical and resemble jellyfish in appearance, but bear 8 longitudinal rows of paddle plates formed by bundles of cilia, which serve as organs of movement. During embryonic development, not two (ectoderm and endoderm), but three germ layers are formed. The third is called mesoderm and then gives rise to muscle tissue. The digestive and nervous systems are more developed than those of cnidarians. Ctenophores are hermaphrodites. There is no alternation of generations among them. One of the largest species, the Belt of Venus, reaches a meter in length, while the diameter of others may not exceed 2 cm. The phylum includes approximately 80 species, divided into two classes: tentacled (Tentaculata) and tentacleless (Atentaculata, or Nuda).
Type flatworms
(Platyhelminthes, from Greek platys - flat, helmins, helminthos - worm). Bilaterally symmetrical animals with more or less pronounced anterior (head) and posterior (tail) ends of the body, dorsal (dorsal) and abdominal (ventral) sides, longitudinal nerve trunks and brain rudiments. At the front end, which during forward movement is the first to come into contact with a new environment, various sensory organs are concentrated. The outer integument is represented by a soft epidermis; the skeleton, circulatory and respiratory systems are absent. The digestive system is not continuous - without an anus, and sometimes completely reduced; There is no secondary body cavity (coelom). The release of decay products occurs with the help of “flame” cells in the form of tubes closed at one end with a bunch of cilia beating inside, which drive the fluid to the excretory tubules and further to the outlet openings. The nervous system consists of an anterior pair of ganglia (clusters of nerve cells) and associated nerve trunks that run along the body. Most are hermaphrodites, i.e. each individual has male and female gonads (testes and ovaries) and their corresponding excretory ducts. Fertilization is internal.
Class flukes, or trematodes
(Cestoidea, from the Greek kestos - belt, ribbon). The flattened, ribbon-shaped body usually consists of segments (hundreds of them in some species up to 12 m long), each of which contains a complete hermaphroditic reproductive system. New segments are formed near the head (scolex) of the worm as a result of continuous budding, so we can say that sexual reproduction is, as it were, combined with asexual reproduction. There is no digestive system - nutrients are absorbed by the entire surface of the body. The head is equipped with various kinds of suction cups and hooks with which the worm is attached from the inside to the wall of the host's intestine.
Type of nemertean
(Nemertini, from the Greek. Nemertes - the name of one of the Nereids, nemertes - infallible). The body is soft, flat, cord-like, not divided into segments, covered with ciliated epithelium. Length from 0.5 cm to 25 m. At the front end, in a special vagina, there is a tubular proboscis that can be thrown out. Dioecious animals with external fertilization, but some species are capable of asexual reproduction by fragmenting the body: from each fragment, as a result of regeneration, a whole worm is formed.
Excretory organs with “flame” cells and the structure of the nervous system bring nemerteans closer to flatworms, but other features, such as a closed circulatory system, allow us to classify them as more advanced forms in an evolutionary sense. In addition, nemerteans differ from flatworms in having a continuous digestive tract with an anus and a simpler reproductive system.
Type of acanthocephala
Acanthocephalans are similar to roundworms (Nematoda), but differ from them in a number of important characteristics, in particular the presence of a proboscis, circular muscles, excretory organs with “flame” cells, a different reproductive system and the absence of a digestive tract. An important difference from all the animals discussed above is the pseudocoel (primary body cavity). 300 species have been described.
Type of rotifer
Rotifers are dioecious, but their males are dwarf, simplified, and in some species they do not exist at all. The most common forms have a very unique reproductive cycle. Their "summer" and "winter" eggs are different. The former are covered with a thin membrane and develop without fertilization; Only females hatch from them, and several generations occur in one season. Finally, for some unknown reason, some females lay small eggs that hatch into males. Mating occurs with internal fertilization. Fertilized “winter” eggs have a thick, dense shell, so they can withstand both frost and drought. When favorable conditions occur, females hatch from them and again lay “summer” eggs. More than 1,300 species of rotifers have been described.
Type gastrociliata
(Gastrotricha, from Greek gaster - stomach, thrix, trichos - hair). Tiny (0.5–1.5 mm) oblong animals that live at the bottom of fresh or salt water bodies. These free-living worms, similar in appearance to ciliated unicellular organisms, are sometimes classified as nematodes. However, they differ from them in the cilia that cover the flattened ventral surface of the colorless and transparent body. The dorsal side is usually convex and bears spines, bristles or scales. In most species the head is discernible, and the rear end is forked or simply tapers to a point; red light-sensitive spots and sensory palps or tentacles are sometimes present. The digestive system is continuous with a muscular pharynx for swallowing small algae - the main food of these worms. Nervous system with a paired cephalic ganglion and lateral trunks stretching along the entire body. The pseudocoel is filled with internal organs; protonephridia with “flame” cells are used for isolation. Characteristically, the tail contains glandular cells that secrete an adhesive substance with which the animal attaches to various objects.
Most of the female's body is occupied by the genitals. The egg is covered with a thick shell with hooks that attach it to solid objects. Development proceeds without larval stages. In freshwater species, only females are known. Forms that live in salt water are hermaphrodites. About 100 species have been described.
Kinorhyncha type
(Kinorhyncha, from Greek kineo - move, rhynchos - snout). Small, almost microscopic marine animals. The head, consisting of two segments, can be retracted into the first two or three segments of the body. There are no cilia, but the body segments bear separate spines, and the head has corollas of them. The body cavity is a pseudo-coel, the digestive system is through. The excretory organs are two tubes, each with a “flame” cell. The nervous system is in contact with the epidermis and includes the anterior dorsal ganglion, the peripharyngeal ring and the abdominal trunk with a ganglion in each segment. The musculature is similar to that known in gastrociliates and rotifers, but is segmented in accordance with the segmented structure of the body. Kinorhynchus are dioecious, but males are usually indistinguishable from females in appearance. Reproductive ducts are present, and fertilization is presumably internal. Approximately 30 species have been described.
Type of priapulid
(Priapulida, from the Greek Priapos - Priapus, the god of fertility, usually depicted with a huge penis). Marine worms living in the cold waters of the North Atlantic, Arctic and Antarctic. They are most similar to kinorhynchs, although their relationship is unclear. The body is cylindrical, approx. long. 10 cm, segmented from the surface and covered with cuticle. The eversible proboscis is covered with spines, also scattered throughout the body. At the posterior end there is a gill-like appendage of unknown purpose. The digestive system is continuous. Priapulids burrow into the mud on the ocean floor, where they prey on other small worms. The excretory organs are protonephridia. Nervous system with perioral ring and ventral nerve trunk without ganglia. All nerve fibers pass through the epidermis. Dioecious animals with external fertilization. Only a few species are known.
Type of roundworms, or nematodes
(Nematoda, from Greek nema, nematos - thread). Unsegmented worms without a proboscis. The body is covered with cuticle, the head is practically not expressed. The digestive tract is through, respiratory and circulatory organs are absent. The body cavity is a pseudo-goal. Muscle fibers are only longitudinal. There are no cilia or flame cells. Nervous system with a peripharyngeal ring, several pairs of cephalic ganglia, as well as dorsal, abdominal and lateral trunks extending to the posterior end of the body. Sensory organs are usually in the form of spines, setae or papillae.
Nematodes, as a rule, are dioecious, with males much smaller than females and distinguished from them by the curved posterior end of the body, the presence of genital papillae and other structures that facilitate mating (copulation). Large females contain up to 1 million eggs and lay up to a quarter of a million of them per day. In freshwater and land species, there are more females than males. The frequent absence of the latter in extensive collections suggests that hermaphroditism among nematodes is much more widespread than is generally believed, although it is quite common among terrestrial forms. In warm, damp soil or in the body of a host organism, the eggs hatch into young worms, similar to adults in everything except for general size and development of the reproductive system.
Type hairy
(Nematomorpha, from Greek nema, nematos - thread, morphe - form). These animals are similar to roundworms in body shape, the presence of a pseudocoel and only longitudinal muscle fibers, as well as in cuticular cover, lack of segmentation, the structure of the nervous and reproductive systems, and even in their lifestyle.
The body length is from 3 to 90 cm, but its diameter rarely exceeds 5 mm. Males have a shorter body than females, and its rear end is bent or curled into a spiral. The cuticle is very thick. The degeneration of the digestive system has gone so far, especially at the oral end, that the worm is unable to swallow food - its pharynx is a dense lump of cells. At the posterior end there is a cloaca - a common excretory tube for digestive waste and reproductive products. In some species, the intestine ends blindly, and then the cloaca is involved only in reproduction. Nervous system with cephalic ganglion, peripharyngeal ring and abdominal trunk; all its parts are closely connected with the epidermis.
Type intrapowder
(Entoprocta, from the Greek entos - inside, proktos - anus). Another name for the phylum is Kamptozoa (bending). A characteristic feature of these animals is that their oral and anal openings are surrounded by a common ring of tentacles on a rounded outgrowth called a lophophore. The tentacles are covered with cilia and push water and food particles into the mouth. All species, with the exception of one, live in the sea either alone or in colonies, attached with a long stalk to solid objects - shells, algae, worms. Body length from 1 to 10 mm. Intraporosacetes are externally similar to bryozoans, i.e. also resemble moss.
The body is not segmented; horseshoe-shaped digestive tract; the excretory organs are protonephridia; the pseudocoelum is filled with a gelatinous mass of cells; the nervous system consists of a ganglion located at the bend of the intestine and nerves extending from it; sensory bristles are present. Some species are dioecious, others are hermaphrodites; Asexual reproduction by budding is very common. 60 species are known.
Type of bryozoan
(Ectoprocta, from the Greek ektos - outside, proktos - anus). This phylum is also known as Bryozoa. It includes animals that are externally similar to intrapowderycetes, but with a true coelom, i.e. peritoneal lining of the body cavity. Unsegmented organisms with a continuous digestive tract; there are no circulatory, respiratory or excretory systems. The anus is located outside the tentacular ring of the lophophore, which explains the Latin name of the group - “Ectoprocta” (“outer-powdery”). The nervous system consists of one ganglion and the nerves extending from it.
The size of individual individuals does not exceed 3 mm, but creeping colonies covering stones, shells, etc. with a thin crust. substrates can occupy an area of more than 1 m2; There are also massive gelatinous colonies, similar to small pumpkins. All bryozoans are hermaphrodites, but sexual reproduction occurs only during a short season. Colonies arise as a result of budding. Freshwater species also form internal buds, protected by a durable shell, the so-called. statoblasts. If the colony dies due to drying out or freezing, the statoblasts survive and give rise to new individuals. Bryozoans live in water, mainly on the dimly lit lower surfaces of various objects. There are two classes.
Class Angiostomata
(Phylactolaema, from Greek phylakto - protect, laemos - throat). The lophophore is horseshoe-shaped, and the lip (epistome) hangs over the mouth opening. Exclusively freshwater forms that form statoblasts.
Naked class
(Gymnolaemata, from Greek gymnos - naked, laemos - throat). The lophophore is ring-shaped, there is no epistome. Most species live in the sea and do not form statoblasts.
Type of cycliophora
(Cycliophora, from the Greek kyklion - circle, wheel; phoros - carrier). In 1991, tiny (0.3 mm) creatures were discovered on the mouth parts of a lobster caught between Denmark and Sweden, which turned out to be representatives of a previously unknown group. Their description was first published in 1995. The name given to these animals is explained by the presence of a fringed, wheel-shaped mouth. The life cycle of cycliophorans is very complex and unusual; it involves mobile non-feeding sexual forms (females and dwarf males), attached feeding asexual forms and two types of larvae. The so-called Pandora larvae develop in an asexual organism, and another asexual form develops inside it. Apparently, bryozoans should be considered the closest relatives of cycliophorans.
Type of phoronid
(Phoronida, from the Greek Phorónis - the name of a nymph). Marine animals with a length of 0.5 to 40 cm. They live alone in secreted tubes, which are immersed with the lower end in silt or sand in shallow sea waters. The edge of the lophophore bears a double row of ciliated tentacles that drive food particles into the mouth.
The vermiform body is unsegmented; all species are hermaphrodites. Muscles are longitudinal and circular; the digestive canal is curved in a horseshoe shape; body cavity - whole; the circulatory system is closed. The nervous system is located not in the epidermis, but under it. The nephridial excretory organs open into two small openings near the anus. There are no special respiratory organs.
Type Brachiopods
(Brachiopoda, from Greek brachion - shoulder, pus, podos - leg). Small solitary animals leading a predominantly sedentary lifestyle in shallow sea waters. The body is protected by a shell, and in appearance they look like bivalves.
Inside the shell are two long spiral “arms” extending from the anterior end of the body, seated along the entire length with tentacles with ciliated cilia - this is a highly expanded lophophore; digestive system through or without anus; Also characteristic are a developed coelom, nephridia, a heart with contractile blood vessels and a peripharyngeal nerve ring. Animals are dioecious; eggs and sperm enter the water from the paired ovaries and testes, where fertilization occurs.
Lockless class
(Inarticulata, from Latin in - not; articulatus - articulated). The shell valves are almost identical, without protrusions and depressions, of which the “lock” holding them together should consist, and without a “beak”, from which in other brachiopods a stalk emerges, which serves for attachment to the substrate; there is an anus.
Castle class
(Articulata). The shell valves (dorsal and ventral) are very different and form a “lock” and a “beak”; digestive system without anus.
Type of molluscs, or soft-bodied
(Mollusca, from Latin mollis - soft). Characteristics common to all these animals: lack of true segmentation; the presence of a thin fold of skin (mantle) secreting the shell; initial bilateral symmetry; through the digestive tract; muscular leg on the ventral side of the body; reduced whole; a special structure in the mouth is the radula (grater), covered with chitinous teeth for scraping food. The nervous system is formed by four pairs of interconnected ganglia, nerves and sensory organs that perceive light, body position in space, smell, tactile stimulation and taste. The heart is located closer to the dorsal side of the body and consists of one or two atria, which receive blood from the body cavity, and a ventricle, which, when contracting, pushes the blood back out. The excretory organs are nephridia.
Based on differences in the processes of reproduction and respiration, types of “legs” and shells, mollusks are divided into six main classes. Representatives of the seventh class Monoplacophora are extremely rare and are known mainly from fossil remains. They have an oval shell, 5-6 pairs of gills and live very deep on the ocean floor.
Class unshelled
(Aplacophora, from Greek a - negation, plako - plate, phoros - carrier). These deep-sea mollusks, also called Solenogastres, are the most primitive. The length of their worm-like body is usually approx. 2.5 cm, but in some forms reaches 30 cm. They differ significantly from other mollusks in the absence of a real leg (it is assumed that a narrow groove along the midline of the ventral surface is homologous to it), a clearly defined head, eyes and tentacles. The body is covered with a cuticle, rather than a shell, which is assumed to have formed later in mollusks.
Class armored
(Polyplacophora, from the Greek polys - many, plako - plate, phoros - load-bearing). These animals, also called chitons, have a flattened, elliptical body, with eight overlapping calcareous plates on the dorsal side, like tiles. Length from 2 mm to 30 cm. The back and sides are covered with a mantle, and most of the lower surface is occupied by a flattened leg. The mouth contains the radula; the respiratory organs are the gills; nervous system with a peripharyngeal ring and two pairs of lateral nerve trunks connected by jumpers (no ganglia). Some species have visual spots. Animals are dioecious; external fertilization. The larvae, as in many of the animal types discussed below, are called trochophores.
Chitons crawl in the sea on rocks and are able to firmly attach to them. If you tear a chiton from a stone, it curls up like a hedgehog, exposing its dorsal plates for protection. Described approx. 750 species.
Class spadefoot or boat-footed
(Scaphopoda, from Greek skaphos - boat, pus, podos - leg). Sea creatures; They live almost completely buried in the bottom silt. The conical shell is thin, elongated and somewhat curved, 5–8 cm long. A pointed leg protrudes from its wide mouth, located in the ground, and the narrow end with a hole at the top protrudes into the water.
Spadefoots breathe with the help of a mantle; they do not have gills. The head is missing. Dioecious animals with external fertilization.
Class Gastropods
(Gastropoda, from Greek gaster - stomach, pus, podos - leg). These animals, which include slugs and snails, are found everywhere: in small ponds and large lakes, in streams and rivers, on mountain tops, in forests and grasslands, on the seabed and in the open ocean. A typical snail has sensory tentacles on its head, two eyes, and a mouth equipped with a radula. The excretory organ is the single kidney. The snail moves with the help of a large mucus-covered leg with nerve ganglia inside. Many terrestrial species breathe with lungs (pulmonary group), others - with gills. Most are hermaphrodites.
The shell of gastropods is sometimes reduced, always single-chambered. Most species are able to completely retract their body into it. The shell is usually conical, twisted into a spiral. In terrestrial slugs it can completely degenerate and is invisible from the outside. In nudibranchs (marine forms whose secondary gills are not covered by anything) no traces of it remain in adulthood. Another marine gastropod, the limpet, has a very flattened shell and looks like an inverted saucer.
Class bivalve
(Pelecypoda, from Greek pelekys - ax, pus, podos - leg). Among these aquatic forms, also called elasmobranchs, everyone knows scallops, mussels, pearl mussels, and oysters. Their shells consist of two more or less identical movably articulated lateral valves. Many species live partially buried in the ground at the bottom of the reservoir, but most crawl, leaving a mark in the form of two furrows (from the edges of the shell) and a slightly loosened strip between them (from a hatchet-shaped foot). Others are completely immersed in the ground, and only long siphons formed by the mantle reach its surface - tubes through which water, and with it food and oxygen, enters the mantle cavity and is then removed from it. Mussels and some other species are firmly attached to stones using secreted threads.
The shell can be closed tightly with the help of one or two closing muscles. Typically, lamellar gills serve as respiratory organs, and at the same time filtering food particles. There is no head or radula.
Bivalves have long been used as food, especially in ancient times. In a number of countries, oyster fishing still flourishes. Pearls are formed in the shells of a number of species: if a foreign body (for example, a grain of sand) gets under the mantle, it surrounds it layer by layer with nacre, and a pearl is obtained. In the past, a lot of damage to pilings and piers was caused by the shipworm, which now makes passages in wood and concrete. About 11,000 modern and even more extinct species of bivalves have been described.
Class Cephalopods
(Cephalopoda, from Greek kephale - head, pus, podos - leg). These marine animals, which include squid, octopus, nautilus and cuttlefish, are considered the most advanced of all mollusks. The large head contains eyes and a mouth with horny jaws and a radula; it is surrounded by either 8 or 10 arms or many tentacles. Dimensions vary from a few centimeters to 8.5 m. All species are dioecious; fertilization is internal. The eggs, surrounded by gelatinous capsules, hatch into miniature, adult-like immature individuals.
Cuttlefish and squid retain a rudimentary shell inside their body; in octopuses it can disappear without a trace. Ships, or nautiluses (one of the orders of cephalopods with 4 modern species - representatives of the same genus), have an external shell; it is curled into a spiral, like that of snails, however, unlike them, it is divided inside by partitions into chambers.
In ancient times, cephalopods were much more numerous and diverse; the number of their species was approaching 10,000, whereas today there are only approx. 400.
Type of sipunculid
(Sipunculida, from Latin siphunculus - pipe). Worm-like marine animals that live in burrows covered inside with mucus. The length of the unsegmented body is from 1 to 50 cm; inside is a vast whole. A mouth bordered with tentacles at the end of a reversible proboscis. There is no skeleton, but all other organ systems are well developed. The animals are dioecious, although males and females do not differ in appearance. The gonads are clearly expressed only during the breeding season. Known approx. 250 species.
Echiurida type
Echiurids are possibly related to sipunculids and priapulids. Described approx. 130 species.
Type annelids
In a number of features of embryonic development, annelids are similar to mollusks. A relationship with arthropods is also revealed based on such characteristics as the structure of the nervous system, the cuticle secreted by the epidermis, and the method of formation of mesoderm; however, ringlets differ from them in the absence of molts and the presence of an extensive coelom. More than 12,000 species have been described, divided into 3 classes.
Class polychaetes
A small group of polychaetes, considered primitive because of their simplified structure, was previously classified as a separate class of primary ringlets (Archiannelida). However, it has now been established that the species included in it are neither primitive nor closely related to each other: their relatively simple organization is explained by their adaptation to life in bottom sediments.
Class oligochaetes
(Oligochaeta, from the Greek oligos - little, chaete - hair). These worms, which also include earthworms, live in water or damp soil. Their body segmentation is well expressed both inside and outside. There is no head or parapodia, but each segment usually bears several pairs of setae. In most species, respiration is cutaneous and there are no gills. Although oligochaetes are hermaphrodites, they do mate. The eggs are fertilized and laid in a cocoon of mucus secreted by the so-called glandular cells. belt on the body. Approximately 3,000 species have been described.
Leech class
(Hirudinea, from Latin hirudo - leech). These worms live in water or damp places on land. The body is flattened. The large posterior sucker serves for attachment; sometimes there is a second - anterior - suction cup. Tentacles, parapodia, and usually bristles are absent. Hermaphrodites, but mating occurs. From eggs surrounded by a cocoon, adult individuals develop, bypassing the larval stage.
Approximately 100 species are known. The length of most of them is from 10 to 85 cm, and the diameter usually does not exceed 2 mm. Depending on the species (only three exceptions are known), the head section (protosome) bears from one to more than 250 tentacles, forming something like a beard, which explains the scientific name of the group.
In the 1970s, three new species were found near sulfur-rich hot springs on the ocean floor. They differ not only in that they live at water temperatures reaching 23° C, but also in their size: length up to 3 m and diameter 35–40 mm; in addition, instead of a beard, they have a feathery plume extending from the head end. It is possible that typical pogonophores absorb nutrients through the body wall, but these giants exist due to the bacteria living in them, which synthesize organic substances from inorganic ones.
Type of pentacle
Type of Tardigrade
(Tardigrada, from Latin tardigradus - slowly moving). This group includes 600 species of animals. Their length is 0.05–1.2 mm; the body consists of four segments bearing a pair of short and thick unarticulated legs. These are pseudocoelomic forms related to annelids and arthropods.
Type of onychophora
(Onychophora, from Greek onyx, onychos - claw, phoros - carrier). These animals, also called primary tracheata (Protracheata), are one of the oldest groups that existed back in the Cambrian, i.e. 500 million years ago. They are similar to warty caterpillars, but are mostly predators, feeding on insects or other small invertebrates. Length ranges from 1.5 to 20 cm. They have two eyes, two fleshy antennae and one pair of jaws. There are from 14 to 43 pairs of legs with paired claws, depending on the species and sex of the animal (males usually have fewer). Onychophorans are dioecious, usually viviparous. They live in damp places; widespread, but mainly in the tropics.
Due to the presence of many similarities with both annelids and arthropods, onychophorans are often called the link between these groups. Like ringlets, they have a segmented body with a soft wall, unsegmented appendages, paired nephridia (excretory tubes) in each segment and an unbranched digestive tract. They are similar to arthropods by tracheal breathing and reduction of the coelom: the space between the internal organs is occupied by the hemocoel, i.e. a large cavity filled with blood (open circulatory system).
Onychophorans are divided into two families with nine genera, the best known of which is Peripata ( Peripatus). Approximately 75 species have been described.
Phylum arthropod
(Arthropoda, from Greek arthron - joint, pus, podos - leg). This is the largest group of animals, uniting, according to various estimates, 1.5–2 million modern and fossil forms. One of the main features that distinguishes it from all more primitive invertebrates is the articulated structure of the limbs. The segmented body consists of a head, thorax and abdomen. Initially, each segment bears a pair of articulated appendages. The external skeleton (exoskeleton) is represented by a dense cuticle; What gives it strength is chitin, an aminopolysaccharide similar in physical properties to horn. The exoskeleton is very weakly extensible, so body growth requires periodic molting, during which the old cover is shed and a new, more spacious one is secreted to replace it. The digestive tract is usually through. The coelom is greatly reduced, and most of the body is occupied by a blood-filled cavity - the hemocoel (an open circulatory system). The nervous system, as well as simple and compound eyes, antennae and other sensory organs are usually well developed.
Arthropods are characterized by dioeciousness and internal fertilization. In some species, eggs develop without fertilization (parthenogenesis). The type is divided into 9 classes.
Class Crustaceans
Sea acorns and barnacles cause great damage by attaching themselves to the bottoms of boats, reducing speed and increasing fuel consumption. Many species are eaten by humans. Much more important, however, is that they serve as food for other animals; Thus, some whales feed almost exclusively on small crustaceans. The number of species reaches 25,000.
Class Labiopods
(Chilopoda, from Greek cheilos - lip, pus, podos - leg). The body is elongated, flattened; on each of the numerous segments of the body there is a pair of legs (hence the common name for these animals - centipedes). The first pair of them are transformed into jaws with poisonous glands and sickle-shaped claws for hunting and protection. On the head there are 3 pairs of jaws, simple ocelli, sometimes forming dense clusters, or compound eyes (some species are eyeless), and antennae. Dioecious, with unpaired gonads. Some species are oviparous, others are viviparous. All lead a terrestrial lifestyle; most live in hot countries and are active at night. Several species are dangerous to humans. Large (up to 25 cm long) labiopods feed on insects and even mice.
Class Dipopods
(Diplopoda, from Greek diploos - double, pus, podos - leg). They are also called centipedes, but they are easily distinguished from labiopods by their more cylindrical body with two pairs of legs on each segment. There are only 2 pairs of jaws. The genital opening is on the third segment (in labiopods - on the penultimate). The length of some species reaches 10 cm. They live in dark, damp places. Approximately 7,000 species are known.
Class sea spiders
(Pycnogonida, from Greek pyknos - thick, gony - knee). The position of this group (also called Pantopoda) within the phylum Arthropods is unclear; sometimes it is classified as arachnid. The body is very small, especially in comparison with the length of the limbs, of which there are usually 7 pairs; the abdomen is greatly shortened. On the head is a proboscis with a mouth opening. There are no respiratory organs. Dioecious; the eggs are carried by the male on specialized legs, where the female wraps them; in most, development proceeds with metamorphosis. Approximately 500 species have been described.
Class Pauropoda
(Pauropoda, from Greek pauros - small, pus, podos - leg). In some systems, symphylos and pauropods are grouped with labiopods and bipopods, respectively. However, pauropods have branched antennae and only 9 or 10 pairs of legs. No eyes. Land animals that live in damp places. More than 100 species are known.
Symphyla class
(Symphyla, from Greek sym - together, phyle - genus, tribe). Small animals (length up to 1 cm) without eyes, but with antennae, 3 pairs of jaws and 12 pairs of legs.
Class insects
(Insecta, from Latin insectum - dissected). All these animals, despite their diversity, have a number of common characteristics. They have three pairs of legs on their chest and usually two pairs of wings (some have only one or none at all). The circulatory system consists of the heart and one artery; there are no veins or capillaries. The respiratory organs are branching tubes - tracheas, which open outwards with spiracles and are suitable for all internal organs. In many larvae, skin respiration plays an important role. The end products of metabolism are absorbed by the blind Malpighian vessels and excreted through them into the hindgut. The nervous system with various sensory organs is well developed. The posterior end of the body usually bears the external genitalia. Fertilization is internal; almost all are dioecious; Some species reproduce parthenogenetically (eggs develop without fertilization). In most species, development proceeds with metamorphosis. Body length – from 0.2 mm to more than 30 cm; Some tropical butterflies have a wingspan exceeding 25 cm.
Insects are abundant in every type of habitat except the ocean. They are the only invertebrates capable of flight. Approximately 900,000 species have been described.
Very few groups of animals have as great an impact on our lives as insects. On the one hand, they serve as carriers of a number of serious diseases and cause enormous harm to agricultural crops, domestic animals and human property, but on the other hand, they bring benefits to humans. They provide, for example, honey, shellac, silk and some dyes. Their role as pollinators of many cultivated plants is invaluable. In addition, many predatory species help control pests. Cm. INSECTS.
Class arachnids
(Arachnida, from Greek arachne - spider). This group includes, among others, spiders, scorpions and ticks; all of them can be easily distinguished from other arthropods by their 4 pairs of legs; the cephalic and thoracic segments are fused together to form the cephalothorax. There are no antennae or real jaws. The first two pairs of modified limbs - chelicerae and pedipalps (literally - claws), and sometimes the first segments of walking legs - allow grabbing and grinding food; When feeding, the animal sucks out only the liquid part of the food. The male is usually smaller than the female; most species are oviparous.
Class Merostomaceae
(Merostomata, from Greek meros - part, stoma - mouth). The oldest marine arthropods. Only 3 genera of horseshoe crabs have survived to this day. The body consists of a fused cephalothorax, covered with a horseshoe-shaped dorsal shield, and an unsegmented abdomen.
Type bristle-jawed
(Chaetognatha, from the Greek chaete - hair, gnathos - jaw). Approximately 115 species of so-called. naval shooters, most of which stay near the surface of the ocean. The type received its name because of the bristles bordering their mouth. The body is translucent, arrow-shaped, non-segmented, without ciliated cover, from 5 mm to 10 cm in length. Other characteristic features: the presence of a head, trunk and caudal sections; through the digestive tract; nervous system with ganglion-bearing peripharyngeal ring, abdominal ganglion and sensory organs. The respiratory, excretory and circulatory systems are absent. Hermaphrodites with internal fertilization; The ovaries are located in the trunk region, the testes are in the caudal region.
The phylogenetic relationships of chaetognaths are not entirely clear, since strongly pronounced adaptations to a predatory lifestyle among plankton mask their relationship with other groups. These are probably highly specialized pseudocoelomic animals, and not degenerated secondary cavities, as some researchers believe.
Type echinoderm
(Echinodermata, from the Greek echinos - hedgehog, derma - skin). Marine animals with a radially symmetrical, non-segmented body without a head and a flexible internal skeleton (endoskeleton) made of calcareous plates. The digestive tract usually ends in the anus, but in some species it is absent; the circulatory system is located in a well-developed coelom. The nervous system is primitive, with a radial structure. Almost all are dioecious; Fertilization occurs in sea water. The ability to restore (regenerate) lost body parts is well developed.
A unique feature of echinoderms is the ambulacral system, which develops from the coelom. It consists of water-filled tubes and is involved in movement, breathing, excretion and nutrition. Lateral branches extend from the radial canals to hundreds of so-called. ambulacral legs on the surface of the body - cylindrical tubes with an extensible ampoule at the base and a suction cup at the free end. By changing the amount of water in the system and contracting the muscles of the legs and ampullae, the animal attaches to the substrate, can crawl and grab food.
Echinoderms are of particular interest because many zoologists consider them to be closely related to hemichordates and chordates. They are similar to representatives of these two types in the method of coelom formation, the formation of mesoderm from the lateral protrusions of the primary intestine, and deuterostome, i.e. the transformation of the blastopore (primary mouth) into the anus and the appearance of the oral opening at the other end of the primary intestine. Most modern echinoderms are crawling animals, but they may have evolved from sessile ancestors. Modern species approx. 5000.
Class holothurians, sea cucumbers, or sea capsules
(Holothuroidea, from the Greek holothurion - aquatic polyp). Marine animals with a cylindrical body similar to a cucumber. The mouth located at its end is surrounded by a corolla of tentacles. The body is soft and leathery to the touch, since the skeleton consists only of microscopic plates. There are no arms or spines, and radial symmetry is manifested only in the equal distances between the five longitudinal rows of legs. There are so-called water lungs formed by branched invagination of the cloaca. They live in shallow waters, where they crawl very slowly along the bottom. Usually dioecious, although males and females are not distinguishable externally. Known approx. 500 species.
Starfish class
(Asteroidea, from Greek aster - star). The body is flattened and looks like a star from above. Most often it has five rays, or arms, but some forms have up to 50; the arms are connected to a central disk, the diameter of which is approximately half their length. Each arm contains gonads and digestive glands, and on its lower surface are rows of ambulacral legs. The surface of the body is hard and rough, because The skeletal plates can be clearly felt. On the aboral (upper) side of the disc there is a madrepore plate - a sieve-like entrance to the system of ambulacral canals; the oral (oral) side is at the bottom. Most species are dioecious; Fertilization is usually external. In some species, the female bears the young in a special chamber under the central disk. Most are predators. Approximately 2000 species have been described.
Class serpentine, or brittle star
(Ophiuroidea, from Greek ophis - snake, ura - tail). Similar in appearance to starfish, there are usually five thin and flexible arms attached to a central disk. Each bears four rows of skeletal plates: aboral (upper), oral (oral, i.e. in this case lower) and two lateral. Only the side rows are prickly. Unlike sea stars, brittle stars have a madrepore plate located on the oral surface of the disc, and the ambulacral legs have lost their motor function and serve as organs of touch. The arms of brittle stars break off easily, but quickly regenerate.
Sea lily class
(Crinoidea, from the Greek krinon - lily). This class unites all living sessile echinoderms (subphylum Pelmatozoa). Their movable rays, or arms, surround the superior oral surface of the body; resembling the long petals of a flower, they give the animal a plant-like appearance. An attachment stalk often extends from below, which appears segmented, because skeletal plates form rings in it. This group is very ancient, existing back in the Cambrian, i.e. 570–510 million years ago. Extinct species approx. 5000, and modern ones are less than 700.
Class sea urchins
(Echinoidea, from the Greek echinos - hedgehog). The body is usually hemispherical or disc-shaped, protected by a solid shell (“shell”) of skeletal plates welded together and covered with movable needles, firmly attached to the shell with their bases. The mouth contains five strong teeth that make up the chewing apparatus (Aristotle's lantern). All animals are dioecious; have 4–5 gonads; external fertilization. Sometimes, especially in cold seas, the juveniles develop in special pouches on the female’s body. Approximately 2000 species are known.
Type hemichordate
(Hemichordata, from Greek hemi - half, chorde - string). Worm-like soft-bodied animals that live at the bottom of the sea. The length of some species reaches 2 m. The body consists of a proboscis, a short collar and an elongated torso. Paired gill slits on the front of the latter and the dorsal nerve trunk indicate proximity to chordates, but there is no third main feature of them - the notochord. The similarity of the larvae covered with cilia - tornaria in hemichordates and bipinnaria in echinoderms - allows us to consider hemichordates as an intermediate link between echinoderms and chordates. There are two classes, including approx. 100 species.
Class enteric-breathing
(Enteropneusta, from Greek enteron - intestine, pneuma - breath). Mobile bottom animals. Dioecious, but one species is also capable of asexual reproduction by transverse division of the body.
Class pterobranchs
(Pterobranchia, from Greek pteron - wing, branchia - gills). Sessile, usually colonial forms. Arms with numerous small tentacles extend from the collar.
Type chordates
(Chordata, from the Greek chorde - string). These secondary cavities are characterized by three main features: 1) a dorsal nerve trunk in the form of a tube; 2) chord, which serves as the axial internal skeleton (endoskeleton); 3) the presence of gill slits at least in the early life stage. The fourth important sign is the heart located on the ventral side of the body. There are three (sometimes four) subtypes.
Subphylum larval chordates, or tunicates
(Urochordata, from Greek ura - tail, chorde - string), or Tunicata (from Latin tunica - shirt-type clothing). Marine animals with a diameter from 1 mm to 40 cm; single or colonial. Some species and all larval stages are free-swimming, but sessile forms are also known. All of them have a body covered with a thick transparent gelatinous membrane - a tunic. Hermaphrodites; Reproduction is sexual or asexual, by budding. There are three classes.
Appendicular class
(Appendicularia, from Latin appendicula - appendage). Free-swimming forms, from 0.3 to 8 cm long, retaining a tail in adulthood; hermaphrodites, reproduction is only sexual; development is direct (no larval stage). Also called Larvacea.
Ascidian class
(Ascidiacea, from Greek askidion - pouch). Solitary and colonial sessile in adult form; in the latter case - with a common tunic. Reproduction is both sexual and asexual - by external budding or the formation of gemmules (internal buds).
Class pelagic tunicates
(Thaliacea, from Greek thaleia - flowering). Free-floating forms. The barrel-shaped body is surrounded by circular muscles; contracting, they push the water entering the body from its rear end, providing forward movement. They reproduce both sexually and by budding, in which one adult animal sometimes forms a chain of developing individuals trailing behind it.
Subphylum cephalochordates
(Cephalochordata, from Greek kefale - head, chorde - string). Representatives of this genus - lancelets - live in the sand in the shallow waters of warm seas. The body is lanceolate with one dorsal and two fin folds located on the sides of the ventral side; tail - behind the anus. Body length up to 10 cm. Dioecious creatures.
Subphylum vertebrates
(Vertebrata, from Latin vertere - to twirl). Vertebrates differ from other chordates in two ways: 1) in most, the notochord is replaced by a segmented (jointed) bone structure called the spine; 2) the brain is protected by a bony cranium, which is why vertebrates are often called craniata (Craniata), in contrast to tunicates and cephalochordates. These are, as a rule, large dioecious animals. They are divided into 7 classes.
Class Cyclostomata
(Cyclostomata, from Greek kyklos - circle, stoma - mouth). These animals, which include hagfishes and lampreys, are the most primitive vertebrates. They are closely related to the scutes (Ostracodermi) of the Devonian period (408–362 million years ago), sometimes called the Age of Pisces; These two groups are combined into the superclass of jawless (Agnatha), opposing all other vertebrates - gnathostomes (Gnathostomata). Cyclostomes have neither jaws nor paired fins. The mouth is in the form of a funnel-shaped sucker with horny teeth for scraping the soft tissues of the animals on which they feed. The body is soft, cylindrical, without scales, covered with mucus; On top of the head there is an unpaired (median) nostril. The heart is two-chambered; cranial nerves 8–10 pairs; the notochord persists throughout life.
Class cartilaginous fish
(Chondrichthyes, from Greek chondros - cartilage, ichthys - fish). Usually these are marine predators - sharks, rays and chimeras. The length of some species reaches 15 m. The skeleton is cartilaginous. The notochord persists throughout life. As a rule, a caudal and paired ventral and pectoral fins are present. The mouth is almost always located on the ventral side. It is armed with jaws with teeth covered with enamel; gill slits 5–7 pairs, heart two-chambered; cranial nerves 10 pairs; two nostrils in front of the mouth; in the lumen of the intestine along its entire length stretches the so-called. spiral valve - a fold that increases the suction area. Tooth-like (placoid) scales make the skin rough.
Cartilaginous fishes are possibly closely related to the extinct armored fishes (Placodermi). Sharks and rays are classified into a subclass of elasmobranchs (Elasmobranchii), contrasted with whole-headed ones (Holocephali), i.e. chimeras.
Class bony fish
(Osteichthyes, from Greek osteon - bone, ichthys - fish). The skeleton is usually bony; most species have thin, flattened scales. The mouth is usually at the anterior end of the body, with well-developed jaws and teeth. The heart is two-chambered. The gills are attached to the gill arches in the lateral gill cavities, covered by a hard operculum. Most species have a swim bladder. There are 10 pairs of cranial nerves.
The sizes are very diverse - from 1 cm to 7 m. This class includes trout, catfish, perch and most other fish inhabiting the planet’s water bodies. Approximately 25,000 species are known.
Class amphibians or amphibians
(Amphibia, from Greek amphi - double, bios - life). Amphibians, which include frogs, toads, salamanders and caecilians, were the first vertebrates with four legs for movement on land (sometimes the legs were lost a second time), and the first to have true lungs that allowed them to breathe air. These are cold-blooded (ectothermic) forms, i.e. their body temperature depends on environmental conditions (like all animals except birds and mammals). The skin is bare, more or less moist, involved in respiration. The heart is three-chambered, consisting of two atria and a ventricle; cranial nerves 10 pairs. With very few exceptions, they are oviparous, with larvae developing in water, and therefore live, as a rule, in damp places near water bodies.
Class reptiles or reptiles
(Reptilia, from Latin repere - crawl). These animals include (in order of complexity of organization) turtles, lizards, snakes and crocodiles. They were the first to fully adapt to life on land: in addition to legs and lungs, they are characterized by: internal fertilization; eggs protected from drying out by a calcareous or leathery shell; dry skin covered with horny scales. There are 12 pairs of cranial nerves. The heart is usually three-chambered (but with a ventricle separated by an incomplete septum), but in crocodiles it is four-chambered, with two atria and two ventricles. During development, special embryonic membranes are formed: amnion, chorion and allantois, therefore reptiles are classified as amniotes, in contrast to the vertebrates discussed above, called anamnias. Modern reptiles are much inferior in size and diversity to their relatives who lived in the Mesozoic era (from 245 to 65 million years ago), which is called the Age of Reptiles.
Bird class
(Aves, from Latin avis - bird). These animals differ from all others by the presence of feathers. They are warm-blooded (endothermic), i.e. body temperature is almost constant regardless of environmental conditions. The front pair of limbs are transformed into wings, although in some species the ability to fly is secondarily lost. Bones are light and usually hollow. There are no teeth, although fossil forms had them. In adult birds, only the right aortic arch is preserved; four-chambered heart; The respiratory organs are the lungs, connected to air sacs located throughout the body. There are 12 pairs of cranial nerves. Fertilization is internal, but there is usually no copulatory organ; all are oviparous. The embryonic membranes are the same as those of reptiles (amniotes); lime eggshell. The sizes are very different - from hummingbirds weighing approx. 3 g to ostriches weighing 130–140 kg. Many species are domesticated, and poultry farming constitutes an important branch of agricultural production. see also BIRDS .
Class mammals or animals
(Mammalia, from Latin mamma - female breast). The characteristic features of these animals are hair (coat) and mammary glands, which serve to feed the offspring. The four limbs are differently specialized depending on the function they perform. Most species have auricles and teeth differentiated into several groups. The respiratory organs are only the lungs, the ventilation of which is facilitated by the diaphragm (the muscular partition between the chest and abdominal cavities). All species are warm-blooded. The heart is a four-chambered heart; In the adult body, only the left aortic arch is preserved. There are 12 pairs of cranial nerves. Fertilization is internal, using the copulatory organ (penis). The embryonic membranes are characteristic of amniotes, and the yolk sac is usually vestigial, because The vast majority of species (except for monotremes - the platypus, echidna and proechidna) are viviparous. Mammals vary greatly in size: from shrews weighing 1.5 g, to whales over 30 m long and weighing up to 120 tons. The number of modern species is 4000.
Since ancient times, observing animals, people have noticed similarities and differences in their structure, behavior, and living conditions. Based on their observations, they divided animals into groups, which helped them understand the system of the living world. Today, man's desire to systematically understand the animal world has become the science of classifying living organisms - taxonomy.
Principles of taxonomy
The foundations of modern taxonomy were laid by the scientists Lamarck and Linnaeus.
Lamarck proposed the principle of relatedness as a basis for assigning animals to one group or another. Linnaeus introduced binary nomenclature, that is, a double name for the species.
Each type in the name has two parts:
- genus name;
- species name.
For example, pine marten. Marten is the name of a genus, which may include many species (stone marten, etc.).
Lesnaya is the name of a specific species.
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Linnaeus also proposed the main taxa, or groups, that we still use today.
View
The species is the initial element of classification.
Organisms are classified as one species according to a number of criteria:
- similar structure and behavior;
- identical set of genes;
- similar ecological living conditions;
- free interbreeding.
The species can be very similar in appearance. Previously, it was believed that the malaria mosquito was one species, but now it has been found that there are 6 species that differ in the structure of their eggs.
Genus
We usually name animals by gender: wolf, hare, swan, crocodile.
Each of these genera may contain many species. There are also genera containing only one species.
Rice. 1. Types of bears.
Differences between species of a genus can be obvious, as between a brown and a polar bear, and completely invisible, as between twin species.
Family
Genera are united into families. The family name may be derived from the generic name, e.g. mustelids or bearish.
Rice. 2. The cat family.
Also, the name of the family can indicate the structural features or lifestyle of animals:
- lamellar;
- bark beetles;
- cocoon worms;
- dung flies.
Related families are collected into orders.
Units
Rice. 3. Order Chiroptera.
For example, the order Carnivores includes animals that are different in structure and lifestyle, such as:
- weasel;
- polar bear;
- fox.
If there is a good harvest of berries and mushrooms, a brown bear from the order of carnivores may not hunt for a long time, while a hedgehog from the order of insectivores hunts almost every night.
Class
Classes are numerous groups of animals. For example, the class of Gastropods has about 93 thousand species, and the class of open-jawed insects has more than a million.
Moreover, new species of insects are discovered every year. According to some biologists, there may be from 2 to 3 million species in this class.
Phylums are the largest taxa. The most important of them:
- chordates;
- arthropods;
- shellfish;
- annelids;
- flatworms;
- roundworms;
- sponges;
- coelenterates.
The most voluminous taxa are kingdoms.
All animals are united in the animal kingdom.
We present the main systematic groups in the table “Classification of Animals”.
Discrepancies
Scientists have different views on the classification of the animal world. Therefore, textbooks often classify a certain group of animals as different taxa.
For example, single-celled animals are sometimes classified as the Kingdom of Protists, and are sometimes considered animals of the protozoan type.
Additional classification elements are often introduced with the prefixes over-, under-, infra-:
- subtype;
- superfamily;
- infraclass and others.
For example, crustaceans were previously considered a class in the phylum Arthropods. In the new books they are considered a subtype.
What have we learned?
The science of taxonomy deals with the classification of species of animals and other organisms. Having studied this topic in 7th grade biology, we learned the main and additional taxa into which lower-order taxa are grouped. Animals are classified according to certain characteristics. The higher the order of the taxon, the more general the characters will be.
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There are many known species of animals, more than two million. They must be placed groups, otherwise it is difficult to understand such diversity. Studying the diversity of animals animal taxonomy. The main task of animal taxonomy is the distribution of animals into groups, that is, their classification. The basic unit of classification is the animal species. By animal species we mean totality organisms or individuals, having a similar structure, lifestyle, capable of interbreeding to form fertile offspring and inhabiting a certain territory. All our domestic dogs, despite their differences, belong to the same mind- Dog. Closely related species of animals are combined into a special group called genus. For example, view Dog and view The wolf is classified as family Wolf. If type there are no closely related animals in nature type, similar to it, it is still distinguished as an independent genus. Close, similar childbirth animals are classified as one family. For example, genus Wolf and genus Raccoon dog included families Wolf, which also includes genus Fox and genus Arctic fox.
Close, similar families merged into squad, squads- V Class, classes- V type, types- V sub-kingdom, subkingdoms- V kingdom. So, family wolf is part of squad predatory, which also includes families felids (e.g. cats, lynx, tiger, leopard, lion), mustelids (e.g. marten, sable, weasel, ferret) and bears (e.g. brown bear, polar bear). Squad predatory is just one of the detachments class mammals, or animals that feed their young with milk. Class mammals are part of type chordates, all representatives of which (fish, amphibians, reptiles, birds, animals) have an internal skeleton - the notochord. Type chordates are just one of the types subkingdoms multicellular animals.
Animal subkingdom
There are only two animal subkingdoms: Protozoa animals, or Unicellular animals, and Multicellular animals. Their main difference is that in protozoa each cell- This independent organism. The cells multicellular animals are included in body and perform various functions: some are protective, others are for obtaining food or digesting it, etc. These cells cannot live outside the body. Sub-kingdoms Unicellular and multicellular organisms make up the animal kingdom. Kingdom animals are distinguished on the basis of characteristics characteristic of all animals:
- nutrition with organic substances, usually living organisms; lack of a dense outer shell in the structure of cells;
- in most cases, mobility and the presence of devices for movement.
Thus, the main systematic groups of animals look like this: kingdom, sub-kingdom, type, Class, squad, family, genus, view. In this scheme kingdom- the highest and largest systematic group of animals, and view- the main small group.
The grouping of animals is not done arbitrarily, but in accordance with scientific data based on detailed study.
The fauna, the classification of which meets the requirements of modern science, amazes with the variety of existing forms. And today scientists continue to discover new species of living creatures living on the planet. It is for this reason that zoology needs an order that takes into account the types of animals. It is the correct classification that allows science to develop and move forward.
Taxonomy
The science of taxonomy helps to navigate the diversity of species that represent the animal world of the Earth. Classifying animals into certain groups is one of the activities she does.
Newly discovered representatives of the animal world must be described by scientists and placed in the place of the general system where they should be located according to the criteria accepted in the scientific world.
The modern system of distributing the animal world into groups is based on determining the degree of their relationship, taking into account the origin, external and internal structure of organisms, and the ability to reproduce offspring. The types of animals are arranged in the table from lowest to highest. This corresponds to the general direction of the evolutionary development of the animal world on Earth.
Type - the basic concept of classification
When defining the concept of species, the body structure of animals, their habitat, and ability to reproduce are taken into account. Groups of individuals with common properties constitute animal species.
The classification of individual species, in turn, can be represented by populations. A feature of a group of animals that are part of a particular population is its relative isolation from representatives of the same species. The formation of such groups of animals is associated with their permanent habitat.
Carl Linnaeus and the classification of animals
There was not always a consensus among scientists about the rules for distributing representatives of living nature into certain groups. The current system was proposed by the famous scientist Carl Linnaeus more than three hundred years ago. It turned out to be very convenient and eliminated confusion in cases where scientists had to describe new species of animals.
The classification used today requires the inclusion of animal names only in Latin. The name contains two words. The first of them indicates the animal’s belonging to the genus and is a noun. The second word in the name must be an adjective and indicate the animal's species.
Such a classification of animals does not allow us to confuse, for example, a black-headed gull with some other species of bird. Among the huge variety of animals, only she has this name.
Genus and family
The group indicating the next systemic unit used when classifying animals is called genus. It brings together closely related species.
An example is the genus of crows, which includes representatives of such species as the jackdaw, crow, and rook. Close genera are grouped into families. For example, there is a widespread family called Corvidae. It included the following genera of birds: raven, jay, magpie, nutcracker.
Units and classes
The entire animal world, the basis of classification of which is species, is united into larger groups. These include squads. For example, the order Passeriformes includes several families: Tits, Swallows, Corvids.
Examples of orders include names such as Passerines, Owls, and Anseriformes. It is easy to guess that orders make up classes of animals. The classification proposed by Carl Linnaeus several centuries ago, refined and improved by modern scientists, rightfully has the status of universal.
Types of animals
All known classes of animals are combined into types. Modern science knows about twenty-five types of animals. An example is the phylum Chordates. It consists of the classes Birds, Mammals, Amphibians. All types known to science constitute the Animal Kingdom. The taxonomy is not arbitrary. All descriptions of animals and their assignment to one group or another in the classification system have a scientific basis. There are also international standards regulating the activities of scientists around the world.
It is necessary to know that the science of taxonomy continues to develop. Today, scientists agree on the identification of such categories as superorder, subtype, subclass in the general system. There is a discussion about what place Unicellular or Protozoa should occupy in the system. In some textbooks, these representatives are classified as a special group representing types of animals.
The classification distinguishes Multicellular organisms into another large group of animals. Among them, the following types are considered: Coelenterates, Flatworms, Roundworms, Annelids. There are other types of animals. For example, the phylum Molluscs is represented by the class Gastropods, Bivalves, Cephalopods. The phylum Arthropods combines the classes Crustaceans, Arachnids, and Insects.
Another numerous type of Chordata includes the classes Cartilaginous and Bony fish, which are united by scientists into the superclass Pisces. Amphibians, Reptiles, Birds and Mammals also belong to the phylum Chordata. In the most highly organized class of mammals, eleven orders are considered. The animal classification scheme proposed by zoologists is not only filled with specific content, but has a scientific basis.