The main stages of animal evolution. Abstract: in biology Stages of animal evolution

high school № 21

Abstract on biology

Stages of animal evolution

I've done the work*

*****************

Teacher: aaaaaaaaaaaaaaaaaa

G. Yakutsk, 2007

Development of life in the Archean era.................................................... 3

Development of life in the Proterozoic and Paleozoic eras.................................5

Development of life in the Mesozoic era.................................................... .....10

Development of life in the Cenozoic era.................................................... ...12

The main stages and directions of evolution of the animal world.

(conclusion) ............................................... ................................................14

Application................................................. ......................................16

Bibliography................................................ ...........................18

Development of life in the Archean era

The history of the evolution of animals has been studied most fully due to the fact that many of them have a skeleton and are therefore better preserved in fossilized remains.

The first living organisms arose in the Archean era. They were heterotrophs and used organic compounds of the “primordial broth” as food. The most important stage in the evolution of life on Earth is associated with the emergence photosynthesis, what caused the division organic world into plant and animal. The first photosynthetic organisms were prokaryotic blue-green algae - cyanea. Cyanes and those that appeared later eukaryotic green algae released free oxygen into the atmosphere from the ocean, which contributed to the emergence of bacteria capable of living in an aerobic environment. Apparently, at the same time, on the border of the Archean and Proterozoic eras, two more major evolutionary events occurred: sexual process And multicellularity. Each new mutation immediately manifests itself in the phenotype. If a mutation is beneficial, it is preserved by selection; if it is harmful, it is eliminated by selection. Haploid organisms continuously adapt to their environment, but they do not develop fundamentally new characteristics and properties. The sexual process dramatically increases the possibility of adaptation to environmental conditions due to the creation of countless combinations in chromosomes. Diploidy, which arose simultaneously with the formed nucleus, allows mutations to be preserved in a heterozygous state and used as reserve of hereditary variability for further evolutionary transformations. In addition, in the heterozygous state, many mutations often increase the viability of individuals and, therefore, increase their chances in the struggle for existence. The emergence of diploidity and genetic diversity of unicellular eukaryotes, on the one hand, led to the heterogeneity of the structure of cells and their association in colonies, on the other hand, the possibility of “division of labor” between the cells of the colony, i.e. formation of multicellular organisms. The division of cell functions in the first colonial multicellular organisms led to the formation of primary tissues - ectoderm And endoderm, differentiated in structure depending on the function performed. Further tissue differentiation created the diversity necessary to expand the structural and functional capabilities of the organism as a whole, resulting in the creation of increasingly complex organs. Improving the interaction between cells - first contact, and then mediated through nervous and endocrine systems– ensured the existence of a multicellular organism as a single whole with a complex and subtle interaction of its parts and a corresponding response to the environment.

The paths of evolutionary transformations of the first multicellular organisms were different. Some switched to a sedentary lifestyle and turned into organisms like sponge Others began to crawl and move along the substrate using cilia. From them came flatworms. Still others retained a floating lifestyle, acquired a mouth and gave rise to coelenterates(see Fig. 1) .

Development of life in the Proterozoic and Paleozoic eras

IN Proterozoic The initial links of animal evolution have not been preserved. IN Proterozoic In sediments, representatives of fully formed types of animals are found: sponges, coelenterates, arthropods.

The fauna in the Paleozoic era developed extremely rapidly and was represented by a large number of diverse forms. Life in the seas flourishes. IN Cambrian period All major animal phyla already exist except chordates. Sponges, corals, echinoderms, mollusks, huge predatory crustaceans - this is an incomplete list of the inhabitants of the Cambrian seas.

IN Ordovician The improvement and specialization of the main types continues. In the geological deposits of this period, the remains of animals that had interior axial skeleton , - jawless vertebrates, the distant descendants of which are modern lampreys and hagfishes. Their gill arches in the course of further evolution turned into jaws lined with teeth. The gill muscles have been transformed into the jaw and sublingual muscles. Thus, on the basis of existing structures - skeletal gill arches, which served as a support for the respiratory organs, a grasping-type oral apparatus arose. Large aromorphosis - appearance of grasping mouthparts- caused a restructuring of the entire organization of vertebrates. The ability to choose food contributed to improved spatial orientation by improving the senses. The first gnathostomes did not have fins and moved in the water using snake-like movements. However, this method of movement turned out to be ineffective when it was necessary to catch moving prey. Therefore, skin folds were important to improve movement in water. In phylogeny, certain sections of this fold develop further and give rise to fins, paired and unpaired. As the size of the folds increased, a skeleton was required to strengthen them. The skeleton arose in the form of a series of cartilaginous (then bone) rays. It is very important that the cartilaginous rays are connected to each other by a cartilaginous plate stretching under the skin along the base of the fins. This plate gave rise to the limb girdle (Fig. 2). Thus, the folds turned into paired pectoral and ventral fins, and the middle part of the fold was reduced. Appearance of paired fins– limbs – the next major aromorphosis in the evolution of vertebrates.

So, jawed vertebrates acquired grasping mouthparts and limbs. In their evolution, they were divided into cartilaginous and bony fish.

IN Silurian period Along with the first land plants, the first air-breathing animals came onto land - arthropods(arachnids). The rapid development of lower vertebrates continued in reservoirs. It is assumed that vertebrates arose in shallow freshwater bodies of water and only then moved to the seas.

IN Devonian vertebrates are represented by three groups: lungfish, ray-finned and lobe-finned fish. At the end of the Devonian, insects appeared (the food supply for future terrestrial vertebrates). Lobe-finned fish were typically aquatic animals, but could breathe atmospheric air using primitive lungs, which were protrusions of the intestinal wall. To understand the further evolution of fish, it is necessary to imagine the climatic conditions in Devonian period. Most of sushi was a lifeless desert. Along the shores of freshwater reservoirs, annelids and arthropods lived in dense thickets of plants. The climate is dry, with sharp temperature fluctuations throughout the day and between seasons. The water level in rivers and reservoirs changed frequently. Many reservoirs dried up completely and froze in winter. Aquatic vegetation died when reservoirs dried up, and plant debris accumulated and then rotted. All this created a very unfavorable environment for fish. Under these conditions, only breathing atmospheric air could save them. Thus, the appearance of lungs can be considered as an idioadaptation to the lack of oxygen in water. When water bodies dried up, animals had two ways of escape: burying in silt or migrating in search of water. The first path was followed by lungfish, whose structure has hardly changed since the Devonian and which now live in small, drying up water bodies in Africa (Fig. 3, A). These fish survive the dry season by burrowing into the mud and breathing atmospheric air. Ray-finned fish had fins supported by separate bony rays. They have spread widely and now represent the largest class of vertebrates in terms of the number of species.

Only lobe-finned fish were able to adapt to life on land. Their fins looked like blades, consisting of separate spines with muscles attached to them. With the help of fins, lobe-finned fish - large animals from 1.5 to several meters in length - could crawl along the bottom. These fish had two main prerequisites for the transition to a terrestrial habitat: muscular limbs and lungs. At the end of the Devonian, lobe-finned fish gave rise to the first amphibians - stegocephals (Fig. 3, B).

Adaptation to life on land required a radical restructuring of the entire organization of animals. The limb from a single elastic plate is transformed into a system of levers separated by joints. The greatest load falls on the hind limb girdle, which becomes much more powerful. The limbs, especially the hind limbs, lengthen. Joints develop between the vertebrae. Lacrimal glands, movable eyelids, and muscles that pull the eyes into the orbit appear; all this protects the cornea from drying out. The lateral muscle segments are divided into a large number of individual muscles that are attached to different parts of the skeleton. Movement on land is associated with the need to increase the mobility of the head, as a result of which in terrestrial vertebrates the skull is separated from the bones of the shoulder girdle. Greater mobility of the limbs is accompanied by separation of the muscles of the shoulder girdle from the lateral muscles of the body and strong development of the abdominal muscles.

Lesson objectives:

1. Educational: generalize and systematize knowledge about the kinship and origin of animals of the main types and classes; show the progressive development of the animal world.
2. Educational: developing the ability to work with information and argue for one’s actions; development logical thinking(based on the discovery of cause-and-effect relationships).
3. Educational: developing the ability to mobilize oneself to do work; education of ecological thinking.

Lesson type: Combined.

During the classes

Goal setting: The “Evolution Timeline” drawing is projected onto the screen (Figure 1).

The process of evolution is complex, mysterious and interesting. Many questions have been answered, but many are still waiting to be answered.

How long did the entire evolution of the animal world take?

It is believed to be about two billion years old. Scientists have proposed the following timeline for clarity. They compare the entire time of development of life on Earth (two billion years) from the period of formation of the cooling crust to a day (24 hours). At this scale the following phenomena occur:

• – hardening earth's crust;
• 12.00 – appearance of the first unicellular organisms;
• 18.30 – appearance of the first fish;
• 19.50 – first attempts of organisms to reach land;
• 20.00 – beginning of the Carboniferous period, ancient amphibians;
• 21.02 – heyday of the age of reptiles;
• 22.10 – first mammals;
• 22.30 – end of the age of reptiles, development of mammals;
• 23.58 – first people.

As we see, out of the total time of 24 hours, a person’s appearance and development takes only one and a half minutes (three million years). And the rest of the time Nature developed without man, before him. How did this development go?

2. Updating basic knowledge. Frontal work.

1. Explain the concept of “evolution of the animal world.”
2. What evidence does paleontology provide for evolution?
3. Give examples of comparative anatomical evidence of evolution.
4. What is the reason for the similarity in the stages of embryonic development of animals of different groups, for example vertebrates?
5. What are the driving forces behind the evolution of species in nature?
6. What did Darwin mean by the expression “struggle for existence”?

3. Explanation of new material.

Teacher's word:

There is a diagram on your desks family tree animal world. (Figure 2)

The diagram shows the evolution of the animal world over more than one billion years. This scheme omits some types of animals: sponges, echinoderms, mammalian lizards, a subphylum of jawless vertebrates and classes: cyclostomes, trilobites and centipedes. While watching the presentation, try to fill in the missing places and write down the types and classes of animals on the diagram.

Additional materials for the presentation (Appendix 1):

Slide 2. The simplest forms of life that first appeared on Earth were primary precellular organisms. From them the next, more highly organized stage of life was formed - primary unicellular organisms. Particularly noteworthy among them are the primary flagellates, which gave rise to the two largest trunks of organic nature: one of them is the plant world, the other trunk is the animal world, which we will talk about today. The ancient forms of colonial unicellular organisms played a decisive role in further evolution.

Slide 3.

All multicellular animals pass through the two-layer embryonic stage in their embryonic development.

Slide 6.

The further development of the animal world is associated with the appearance of the first three-layer animals, similar to primitive free-living ciliated worms and descended from ancient primitive two-layer animals. Three-layer animals received in the process historical development progressive structural features: muscular system and parenchyma. The appearance of muscles ensured faster and more perfect movement of animals, and thanks to the parenchyma, the internal environment of the body was formed, providing a more perfect metabolism. The first three-layered animals include types of flatworms and roundworms.

Slide 12.

Representatives of arachnids already in the Devonian adapted to a terrestrial lifestyle. They developed air breathing organs (lungs, trachea). These were the first land animals. Millipedes and insects are typical terrestrial animals. The insects appear to have evolved from millipede-like ancestors. This is the highest class among invertebrates, achieving a very high organization. Insects have adapted to flight and are extremely diverse.

Slide 15.

Chordates are characterized by the formation of a dorsal string, or notochord, during embryonic development. For some, it remains in this form throughout their lives, for others it is replaced by a cartilaginous or bone spine. Lancelet is of great interest for understanding the phylogeny of chordates. This is, as it were, a living diagram of the structure and embryonic development of chordates (the presence of a notochord, a neural tube, an intestine with the anterior part transformed into a gill section and with a characteristic hepatic outgrowth, the circulatory system, the nature of fragmentation of a fertilized egg, the formation of three germ layers, the process of organogenesis, etc.). d.).

Slide 17.

In everyday life, cyclostomes are classified as fish, although they differ sharply from fish in the absence of jaws and many other features of a more primitive organization.

Slide 19.

TO cartilaginous fish include sharks, rays and chimeras.

4. Reflection.

Find on the diagram of the development of the animal world the types of invertebrate animals and chordates (classes of fish, amphibians, reptiles, birds, mammals). Explain the difference in their different arrangement in the diagram.

1. Which animals are considered the most ancient?
2. From which animals did multicellular animals originate?
3. From which animals did three-layered animals originate?
4. How was the structure of chordates made more complex?

At the end of the lesson, students hand in workbooks with pedigree diagrams of the development of the animal world supplemented in them and receive homework from the textbook.

Bibliography:

1. Bykhovsky B.E., Kozlova E.V., Monchadsky A.S. and others. Biology: Animals.
2. Textbook for 7-8 grades. secondary school./ Ed. Kozlova M.A. – M.: “Enlightenment”, 1990.
3. Pepelyaeva O.A., Suntsova I.V. Biology 7 (8) grade. Universal lesson developments. – M.: VAKO, 2006. – 432s. - (To help the school teacher).

Nikishov A.N., Sharova I.Kh. Biology: Animals: Textbook. For 7-8 grades general education. textbook Establishments. – M.: Education, 1994. – 256 p. This document is a description of a lesson made in the form of a presentation using Microsoft applications (

Power Point

Abstract on biology

Stages of animal evolution

I've done the work*

*****************

Teacher: aaaaaaaaaaaaaaaaaa

G. Yakutsk, 2007

Development of life in the Archean era.................................................... 3

Development of life in the Proterozoic and Paleozoic eras.................................5

Development of life in the Mesozoic era.................................................... .....10

Development of life in the Cenozoic era.................................................... ...12

The main stages and directions of evolution of the animal world.

(conclusion) ............................................... ................................................14

Application................................................. ......................................16

Bibliography................................................ ...........................18

Development of life in the Archean era

The history of the evolution of animals has been studied most fully due to the fact that many of them have a skeleton and are therefore better preserved in fossilized remains.

The first living organisms arose in the Archean era. They were heterotrophs and used organic compounds of the “primordial broth” as food. The most important stage in the evolution of life on Earth is associated with the emergence photosynthesis, which led to the division of the organic world into plant and animal. The first photosynthetic organisms were prokaryotic blue-green algae - cyanea. Cyanes and those that appeared later eukaryotic green algae released free oxygen into the atmosphere from the ocean, which contributed to the emergence of bacteria capable of living in an aerobic environment. Apparently, at the same time, on the border of the Archean and Proterozoic eras, two more major evolutionary events occurred: sexual process And multicellularity. Each new mutation immediately manifests itself in the phenotype. If a mutation is beneficial, it is preserved by selection; if it is harmful, it is eliminated by selection. Haploid organisms continuously adapt to their environment, but they do not develop fundamentally new characteristics and properties. The sexual process dramatically increases the possibility of adaptation to environmental conditions due to the creation of countless combinations in chromosomes. Diploidy, which arose simultaneously with the formed nucleus, allows mutations to be preserved in a heterozygous state and used as reserve of hereditary variability for further evolutionary transformations. In addition, in the heterozygous state, many mutations often increase the viability of individuals and, therefore, increase their chances in the struggle for existence. The emergence of diploidity and genetic diversity of unicellular eukaryotes, on the one hand, led to the heterogeneity of the structure of cells and their association in colonies, on the other hand, the possibility of “division of labor” between the cells of the colony, i.e. formation of multicellular organisms. The division of cell functions in the first colonial multicellular organisms led to the formation of primary tissues - ectoderm And endoderm, differentiated in structure depending on the function performed. Further tissue differentiation created the diversity necessary to expand the structural and functional capabilities of the organism as a whole, resulting in the creation of increasingly complex organs. Improving the interaction between cells - first contact, and then mediated through nervous and endocrine systems– ensured the existence of a multicellular organism as a single whole with a complex and subtle interaction of its parts and a corresponding response to the environment.

The paths of evolutionary transformations of the first multicellular organisms were different. Some switched to a sedentary lifestyle and turned into organisms like sponge Others began to crawl and move along the substrate using cilia. From them came flatworms. Still others retained a floating lifestyle, acquired a mouth and gave rise to coelenterates(see Fig. 1) .

Development of life in the Proterozoic and Paleozoic eras

IN Proterozoic The initial links of animal evolution have not been preserved. IN Proterozoic In sediments, representatives of fully formed types of animals are found: sponges, coelenterates, arthropods.

The fauna in the Paleozoic era developed extremely rapidly and was represented by a large number of diverse forms. Life in the seas flourishes. IN Cambrian period All major animal phyla already exist except chordates. Sponges, corals, echinoderms, mollusks, huge predatory crustaceans - this is an incomplete list of the inhabitants of the Cambrian seas.

IN Ordovician The improvement and specialization of the main types continues. In the geological deposits of this period, the remains of animals that had internal axial skeleton, - jawless vertebrates, the distant descendants of which are modern lampreys and hagfishes. Their gill arches in the course of further evolution turned into jaws lined with teeth. The gill muscles have been transformed into the jaw and sublingual muscles. Thus, on the basis of existing structures - skeletal gill arches, which served as a support for the respiratory organs, a grasping-type oral apparatus arose. Large aromorphosis - appearance of grasping mouthparts- caused a restructuring of the entire organization of vertebrates. The ability to choose food contributed to improved spatial orientation by improving the senses. The first gnathostomes did not have fins and moved in the water using snake-like movements. However, this method of movement turned out to be ineffective when it was necessary to catch moving prey. Therefore, skin folds were important to improve movement in water. In phylogeny, certain sections of this fold develop further and give rise to fins, paired and unpaired. As the size of the folds increased, a skeleton was required to strengthen them. The skeleton arose in the form of a series of cartilaginous (then bone) rays. It is very important that the cartilaginous rays are connected to each other by a cartilaginous plate stretching under the skin along the base of the fins. This plate gave rise to the limb girdle (Fig. 2). Thus, the folds turned into paired pectoral and ventral fins, and the middle part of the fold was reduced. Appearance of paired fins– limbs – the next major aromorphosis in the evolution of vertebrates.

So, jawed vertebrates acquired grasping mouthparts and limbs. In their evolution, they were divided into cartilaginous and bony fish.

IN Silurian period Along with the first land plants, the first air-breathing animals came onto land - arthropods(arachnids). The rapid development of lower vertebrates continued in reservoirs. It is assumed that vertebrates arose in shallow freshwater bodies of water and only then moved to the seas.

IN Devonian vertebrates are represented by three groups: lungfish, ray-finned and lobe-finned fish. At the end of the Devonian, insects appeared (the food supply for future terrestrial vertebrates). Lobe-finned fish were typically aquatic animals, but could breathe atmospheric air using primitive lungs, which were protrusions of the intestinal wall. To understand the further evolution of fish, it is necessary to imagine the climatic conditions in the Devonian period. Most of the land was lifeless desert. Along the shores of freshwater reservoirs, annelids and arthropods lived in dense thickets of plants. The climate is dry, with sharp temperature fluctuations throughout the day and between seasons. The water level in rivers and reservoirs changed frequently. Many reservoirs dried up completely and froze in winter. Aquatic vegetation died when reservoirs dried up, and plant debris accumulated and then rotted. All this created a very unfavorable environment for fish. Under these conditions, only breathing atmospheric air could save them. Thus, the appearance of lungs can be considered as an idioadaptation to the lack of oxygen in water. When water bodies dried up, animals had two ways of escape: burying in silt or migrating in search of water. The first path was followed by lungfish, whose structure has hardly changed since the Devonian and which now live in small, drying up water bodies in Africa (Fig. 3, A). These fish survive the dry season by burrowing into the mud and breathing atmospheric air. Ray-finned fish had fins supported by separate bony rays. They have spread widely and now represent the largest class of vertebrates in terms of the number of species.

Only lobe-finned fish were able to adapt to life on land. Their fins looked like blades, consisting of separate spines with muscles attached to them. With the help of fins, lobe-finned fish - large animals from 1.5 to several meters in length - could crawl along the bottom. These fish had two main prerequisites for the transition to a terrestrial habitat: muscular limbs and lungs. At the end of the Devonian, lobe-finned fish gave rise to the first amphibians - stegocephals (Fig. 3, B).

Adaptation to life on land required a radical restructuring of the entire organization of animals. The limb from a single elastic plate is transformed into a system of levers separated by joints. The greatest load falls on the hind limb girdle, which becomes much more powerful. The limbs, especially the hind limbs, lengthen. Joints develop between the vertebrae. Lacrimal glands, movable eyelids, and muscles that pull the eyes into the orbit appear; all this protects the cornea from drying out. The lateral muscle segments are divided into a large number of individual muscles that are attached to different parts of the skeleton. Movement on land is associated with the need to increase the mobility of the head, as a result of which in terrestrial vertebrates the skull is separated from the bones of the shoulder girdle. Greater mobility of the limbs is accompanied by separation of the muscles of the shoulder girdle from the lateral muscles of the body and strong development of the abdominal muscles.

For Carboniferous period stegocephalians lived, fed and reproduced in water. They crawled onto land, but did not make any significant migrations. Stegocephalians divided (diverged) into a large number of forms - from large fish-eating predators to small ones that fed on invertebrates. On land, stegocephalians had no enemies, and there was abundant food - worms, arthropods that reached large sizes (Fig. 3, B). Many groups of amphibians transitioned to life on land and returned to the water only to reproduce.

IN Permian period There was an uplift of land, as well as drying and cooling of the climate. Amphibians are becoming extinct due to deterioration climatic conditions, and due to extermination by mobile predatory reptiles. Even in the Carboniferous, among the stegocephalians, a group stood out that had well-developed limbs and a mobile system of the first two vertebrae (Fig. 3, D - F). Representatives of the group reproduced in water, but went further on land than amphibians, feeding on land animals, and then plants. This group was named cotylosaurs. Later, reptiles and mammals evolved from them.

Reptiles acquired properties that allowed them to finally break their connection with the aquatic environment. Internal fertilization and accumulation of yolk in the egg made reproduction on land possible. Skin keratinization and more complex structure the kidneys contributed to a sharp reduction in water loss by the body and widespread dispersal. The chest provided a more efficient type of breathing - suction. Lack of competition caused wide use reptiles on land and returning some of them to the aquatic environment.

Development of life in the Mesozoic era

At the beginning of the next Mesozoic era mountain-building processes take place on Earth. The Urals, Tien Shan, and Altai appear. For the most part globe A warm climate close to the modern tropical climate is established. By the end of the Mesozoic era, the zone of dry climatic conditions expanded, and the areas of seas and oceans decreased. IN Triassic, In the animal world, insects and reptiles flourish. Reptiles occupy a dominant position and are represented by a large number of forms (Fig. 14.4)

IN Jurassic period Flying lizards appear and conquer the air. IN Cretaceous period The specialization of reptiles continues, they reach enormous sizes. The mass of some of them (dinosaurs) reached 50 tons. end of the Cretaceous mountain-building processes occur again. The Alps, Andes, and Himalayas appear. Cooling sets in and the range of near-water vegetation shrinks. Herbivores are dying out, followed by carnivorous dinosaurs. Large reptiles are preserved only in the tropical zone (crocodiles). Due to extinction predatory reptiles The most adaptable animals are warm-blooded animals, birds and mammals. Many forms of invertebrates and sea lizards are dying out in the seas.

Birds evolved from fully formed reptiles - archosaurs. The emergence of birds was accompanied by the appearance of large aromorphoses in their structure: they lost one of the two aortic arches and acquired a complete septum between the right and left ventricles of the heart. The complete separation of arterial and venous blood flow caused the birds to be warm-blooded. In other features of their organization they are similar to reptiles, and they are sometimes called “feathered reptiles.” All the structural features of birds - feather cover, the transformation of the forelimbs into wings, a horny beak, air sacs and double breathing, shortening of the hindgut - are adaptations to flight, i.e. idioadaptations .

The emergence of mammals is associated with a number of large aromorphoses that developed in representatives of one of the subclasses of reptiles. The aromorphoses that determined the formation of mammals as a class include: the formation of hair and a four-chambered heart, complete separation of arterial and venous blood flows, intrauterine development of the offspring and feeding the baby with milk. Carrying embryos in the mother's body and caring for the offspring dramatically increased the survival rate of mammals. Aromorphoses should also include the development of the cerebral cortex, which determined the predominance of conditioned reflexes over unconditioned ones and the possibility of adaptation to unstable environmental conditions by changing behavior. Mammals arose in the Triassic (Fig. 14.5), but could not compete with carnivorous dinosaurs throughout 100 million years occupied a subordinate position.

Development of life in the Cenozoic era

At first Cenozoic era The mountain-building processes that began at the end of the Mesozoic are completed. The Mediterranean, Black, Caspian and Aral seas are separated. A warm, uniform climate is established. In the Quaternary period of the Cenozoic era (2-3 million years ago), glaciation of a significant part of the Earth began. The ice cover reached an average of 57 o N, reaching 40 o N in some areas.

Development of the animal world in the Cenozoic era characterized by further differentiation of insects, intensive speciation in birds and extremely rapid progressive development of mammals.

Mammals are represented by three subclasses: monotremes (platypus and echidna), marsupials and placentals. Monotreme, or oviparous, mammals arose independently of other mammals back in Jurassic period from animal-like reptiles. Marsupials and placental mammals descended from a common ancestor in Cretaceous period and coexisted until the onset of the Cenozoic era, when there was an “explosion” in the evolution of placentals, as a result of which these mammals displaced marsupials from most continents.

The most primitive were insectivorous mammals, from which the first carnivores and primates descended. Ancient carnivores gave rise to ungulates. IN Paleogene mammals begin to conquer the sea (cetaceans, pinnipeds, etc.). TO end of the Neogene All modern families of mammals are already found. One of the groups of monkeys - Australopithecus - became the ancestor of the branch leading to the genus Man.

Glaciations Quaternary period, which reached their maximum distribution about 250 thousand years ago, contributed to the development of cold resistance of the fauna. In the Northern Caucasus and Crimea there were mammoths, woolly rhinoceroses, reindeer, arctic foxes, and polar partridges. The formation of large masses of ice caused a decrease in the level of the World Ocean. This is a decrease in different periods was 85-120 m compared to the modern one. As a result, the continental shoals of North America and Northern Eurasia were exposed. Land “bridges” appeared that connected the North American continent with the Eurasian continent (at the site of the current Bering Strait), the British Isles with the European continent, etc. Migration of species took place along such “bridges,” which led to the formation of the fauna of the continents that is modern to us. Climate changes in the Quaternary period of the Cenozoic era influenced the evolution of human ancestors.

The main stages and directions of evolution of the animal world.

(conclusion)

Multicellular animals descend from unicellular organisms through colonial forms. The first animals were probably coelenterates. Ancient coelenterates gave rise to flatworms - animals with bilateral symmetry.

From ancient ciliated worms, the first secondary cavities arose - annelids. Ancient marine polychaetes probably served as the basis for the emergence of the types of arthropods, molluscs and chordates.

The oldest traces of animals date back to the Precambrian (about 700 million years ago). In the Cambrian and Ordovician periods Sponges, coelenterates, worms, echinoderms, trilobites predominate, and mollusks appear.

In the Ordovician, jawless armored fish appeared, and then jawed fish. Most of these animals are characterized by the presence of bilateral symmetry, a body cavity, an external (arthropod) or internal (walking) hard skeleton, a progressive ability to active movement, separation of the anterior end of the body with the mouth opening and sensory organs, gradual improvement of the central nervous system.

The first gnathostomes gave rise to ray-finned and lobe-finned fish. Supporting elements in the fins later developed the limbs of terrestrial vertebrates. The most important aromorphoses in this line of evolution are the development of movable jaws from the gill arches, the development from skin folds fins, and then the formation of girdles of paired pectoral and abdominal limbs. Lungfish and lobe-finned fish could breathe atmospheric oxygen through swim bladders connected to the esophagus and equipped with a system of blood vessels.

The first land animals, stegocephalians, originated from lobe-finned fish. Stegocephalians were divided into several groups of amphibians, which reached their peak in the Carboniferous. The first vertebrates reached land by transforming their fins into limbs. ground type, air bubbles - into the lungs.

Truly terrestrial animals - reptiles, which conquered land towards the end - originate from amphibians Permian period. The development of land by reptiles ensured the presence of dry, keratinized integuments, internal fertilization, a large amount of yolk in the egg, protective shells of eggs that protect embryos from drying out and other environmental influences. Among the reptiles, a group of dinosaurs stood out, which gave rise to mammals. The first mammals appeared in the Triassic period of the Mesozoic era. Later, also from one of the branches of reptiles, toothed birds (Archaeopteryx) evolved, and then modern birds. Birds and mammals are characterized by such features as warm-bloodedness, a four-chambered heart, one aortic arch (creating a complete separation of the large and small circles of blood circulation), intensive metabolism - features that ensured the flourishing of these groups of organisms.

At the end of the Mesozoic, placental mammals appeared, for which the main progressive features were the appearance of the placenta and intrauterine development of the fetus, feeding the young with milk, and a developed cerebral cortex. At the beginning of the Cenozoic era, a detachment of primates separated from insectivores, the evolution of one of the branches of which led to the emergence of humans.

Parallel to the evolution of vertebrates was the development of invertebrate animals. Transition from water to terrestrial environment habitat was realized in arachnids and insects with the development of a perfect solid exoskeleton, articulated limbs, excretory organs, nervous system, sensory organs and behavioral reactions, the appearance of tracheal and pulmonary respiration. Among mollusks, access to land was observed much less frequently and did not lead to the diversity of species that is observed in insects.

Main features of the evolution of the animal world:

· progressive development of multicellularity and, as a consequence, specialization of tissues and all organ systems;

· free lifestyle, which determined the development of various behavioral mechanisms, as well as the relative independence of ontogenesis from fluctuations in factors environment;

· the appearance of a hard skeleton: external in some invertebrates (arthropods) and internal in chordates;

· progressive development of the nervous system, which was the basis for the emergence of conditioned reflex activity, the development of social behavior in different groups highly organized animals.

The accumulation of a number of large aromorphoses in the process of biological evolution led to a qualitative leap - the social form of the movement of matter and the emergence of human society. The main directions of animal evolution are shown in Fig. 1.

Application

Rice. 1. The main stages of the evolution of eukaryotic organisms

Fig.2. Skeleton of the paired fin of lobe-finned fish and stegocephalus:

A- shoulder girdle and fin of lobe-finned fish; B - internal skeleton fin;

IN- skeleton of the forelimb of a stegocephalus.

1 – an element homologous to the humerus; 2 – element homologous to the radius;

3 – element homologous to the ulna; 4, 5, 6 - wrist bones, 7 - phalanges of fingers

Fig.3. Animals Paleozoic era:

A- lungfish; B- stegocephalus; IN- a giant dragonfly-like insect;

G - E- the oldest reptiles

Fig.4. Reptiles of the Mesozoic era:

A- horned dinosaur; B- ichthyosaur; IN- flying tailed lizard; G- brontosaurus;

D, F- flying tailless lizards; E- stegosaurus;

Bibliography

1. Zakharov V.B., Mamontov S.G., Sivoglazov V.I.

Biology: general patterns: Textbook for 10-11 grades. general education institutions. – M.: Shkola-Press, 1996. – 624 p.: ill.

2. Iordansky N.N.

Evolution of life: Textbook. aid for students higher ped. textbook institutions - M.: Publishing center "Academy", 2001 - 432 p.

3. General biology: Textbook for 11th grade 11-year-old secondary school, for basic and advanced levels. N.D. Lisov, L.V. Kamlyuk, N.A. Lemeza et al. Ed. N.D. Lisova. – Mn.: Belarus, 2002.- 279 p.

EUKARYOTES– organisms (all except bacteria, including cyanobacteria) that have a formed cell nucleus, delimited from the cytoplasm by a nuclear envelope. Genetic material is contained in chromosomes. Eukaryotic cells have mitochondria, plastids and other organelles. Characteristic of the sexual process

Diploidy- the presence in the nucleus of a plant or animal cell of a paired set of chromosomes.

ECTODERM– The outer layer of the embryo of multicellular animals and humans in the early stages of its development.

ENDODERM– The inner layer of the embryo of multicellular animals and humans in the early stages of its development.

See Appendix

COTYLOSAURUS [< Greek kotylē cup, cup + ... saurus ].fell. Representative of the class of the most ancient (second half Paleozoic) and primitive reptiles . | The name reflects the cup-shaped facets of the vertebrae.

Eukaryotic organisms specializing in heterotrophic nutrition gave rise to Animals and Fungi.

IN Proterozoic era All known types of multicellular invertebrate animals arise. There are two main theories about the origin of multicellular animals. According to the theory of gastrea (E. Haeckel), the initial method of formation of a two-layer embryo is invagination (invagination of the wall of the blastula). According to the theory of phagocytella (I.I. Mechnikov), the initial method of formation of a two-layer embryo is immigration (movement of individual blastomeres into the cavity of the blastula). Perhaps these two theories complement each other.

Coelenterates are representatives of the most primitive (two-layer) multicellular organisms: their body consists of only two layers of cells: ectoderm and endoderm. The level of tissue differentiation is very low.

In the Lower Worms (Flat and Roundworms) the third germ layer appears - the mesoderm. This is a major aromorphosis, due to which differentiated tissues and organ systems appear.

Then evolutionary tree animals branches into Protostomes and Deuterostomes. Among Protostomes, Annelids form a secondary body cavity (coelom). This is a major aromorphosis, thanks to which it becomes possible to divide the body into sections.

Annelids have primitive limbs (parapodia) and homonomic (equivalent) body segmentation. But at the beginning of the Cambrian, arthropods appeared, in which parapodia were transformed into articulated limbs. In Arthropods, heteronomous (unequal) segmentation of the body appears. They have a chitinous exoskeleton, which contributes to the appearance of differentiated muscle bundles. The listed features of Arthropods are aromorphoses.

The most primitive arthropods - Trilobites - dominated the Paleozoic seas. Modern gill-breathing primary aquatic arthropods are represented by Crustaceans. However, at the beginning of the Devonian (after plants reached land and the formation of terrestrial ecosystems), Arachnids and Insects reached land.

Insects are most adapted to life on land, thanks to the appearance of large aromorphoses:

– The presence of embryonic membranes – serous and amniotic.

– Presence of wings.

– Plasticity of the oral apparatus.

With the appearance of flowering plants in the Cretaceous period, co-evolution Insects and Flowers (coevolution), and they form joint adaptations (coadaptations). In the Cenozoic era, insects, like flowering plants, are in a state of biological progress.

Among Deuterostome animals, chordates reach their highest peak, in which a number of large aromorphoses appear: notochord, neural tube, abdominal aorta (and then the heart).

The first Vertebrates (Jawless) originated from primitive chordates in the Silurian. In vertebrates, the axial and visceral skeleton is formed, in particular, the braincase and jaw region of the skull, which is also an aromorphosis. Lower gnathostome vertebrates are represented by a variety of fish. Modern classes of fish (Cartilaginous and Bony) were formed at the end of the Paleozoic - beginning of the Mesozoic).

Part bony fish(Fleshy-lobed), thanks to two aromorphoses - pulmonary respiration and the appearance of real limbs - gave rise to the first Quadrupeds - Amphibians (Amphibians). The first amphibians came onto land in the Devonian period, but their heyday occurred in the Carboniferous period (numerous stegocephals). Modern amphibians appear at the end of the Jurassic period.

In parallel, among the Quadrupeds, organisms with embryonic membranes appear - Amniotes. The presence of embryonic membranes is a large aromorphosis that first appears in Reptiles. Thanks to the embryonic membranes, as well as a number of other features (keratinizing epithelium, pelvic buds, appearance of the cerebral cortex), Reptiles have completely lost their dependence on water. The appearance of the first primitive reptiles - cotylosaurs - dates back to the end of the Carboniferous period. In the Permian, various groups of reptiles appeared: beast-toothed, proto-lizards and others. At the beginning of the Mesozoic, branches of turtles, plesiosaurs, and ichthyosaurs were formed. Reptiles begin to flourish.

Two branches are separated from groups close to the proto-lizards evolutionary development. One branch at the beginning of the Mesozoic gave rise to a large group of pseudosuchians. Pseudosuchia gave rise to several groups: crocodiles, pterosaurs, ancestors of birds and dinosaurs, represented by two branches: lizards (Brontosaurus, Diplodocus) and ornithischians (only herbivorous species - Stegosaurus, Triceratops). The second branch at the beginning of the Cretaceous period led to the emergence of a subclass of squamates (lizards, chameleons and snakes).

However, the Reptiles could not lose their dependence on low temperatures: Warm-bloodedness is impossible for them due to the incomplete division of blood into venous and arterial. At the end of the Mesozoic, with climate change, a mass extinction of reptiles occurred.

Only in some pseudosuchians in the Jurassic period does a complete septum between the ventricles appear, the left aortic arch is reduced, a complete separation of the circulatory circles occurs, and warm-bloodedness becomes possible. Subsequently, these animals acquired a number of adaptations to flight and gave rise to the class Birds.

IN Jurassic deposits Mesozoic era (≈ 150 million years ago) prints of the First Birds were discovered: Archeopteryx and Archaeornis (three skeletons and one feather). They were probably arboreal climbing animals that could glide but were not capable of active flight. Even earlier (at the end of the Triassic, ≈ 225 million years ago) protoavis existed (two skeletons were discovered in 1986 in Texas). The skeleton of Protoavis was significantly different from the skeleton of reptiles; the cerebral hemispheres and cerebellum were increased in size. During the Cretaceous period, there were two groups of fossil birds: Ichthyornis and Hesperornis. Modern groups of birds appear only at the beginning of the Cenozoic era.

A significant aromorphosis in the evolution of birds can be considered the appearance of a four-chambered heart in combination with a reduction of the left aortic arch. There was a complete separation of arterial and venous blood, which made possible further development of the brain and a sharp increase in the level of metabolism. The flourishing of Birds in the Cenozoic era is associated with a number of major idioadaptations (the appearance of feathers, specialization of the musculoskeletal system, development of the nervous system, caring for offspring and the ability to fly), as well as with a number of signs of partial degeneration (for example, loss of teeth).

At the beginning of the Mesozoic era, the first Mammals appeared, which arose due to a number of aromorphoses: enlarged forebrain hemispheres with a developed cortex, a four-chambered heart, reduction of the right aortic arch, transformation of the suspension, quadrate and articular bones into auditory ossicles, the appearance of fur, mammary glands, differentiated teeth in the alveoli, preoral cavity.

In the Jurassic period of the Mesozoic era, Mammals were represented by at least five classes (Multitubercles, Tritubercles, Tricodonts, Symmetrodonts, Panthotheriums). One of these classes probably gave rise to the modern Protobeasts, and the other to the Marsupials and Placentals. Placental mammals, thanks to the appearance of the placenta and true viviparity, enter a state of biological progress in the Cenozoic era.

The original order of Placentals are Insectivores. Early on, the Insectivores were separated from the Incomplete Teeth, Rodents, Primates and the now extinct group of Creodonts - primitive predators. Two branches separated from the Creodonts. One of these branches gave rise to modern Carnivores, from which Pinnipeds and Cetaceans separated. Another branch gave rise to primitive ungulates (Condylarthra), and then to the Odd-toed, Artiodactyl and related orders.

The final differentiation of modern groups of Mammals was completed during the era of great glaciations - in the Pleistocene. The modern species composition of Mammals is significantly influenced by the anthropogenic factor. In historical times, the following species were exterminated: aurochs, Steller's cow, tarpan and other species.

At the end of the Cenozoic era, some Primates experienced a special type of aromorphosis - overdevelopment of the cerebral cortex. As a result, there is completely the new kind organisms – Homo sapiens.

The evolutionary development of living beings should be considered as a holistic process of development of the living population of the biosphere from the initial, primitive forms to the modern, most advanced. This applies equally to morphological and biochemical structures and physiological processes.

One of most interesting questions in the evolution of organisms - the origin of multicellularity. No one doubts that multicellular organisms evolved from unicellular organisms, and most scientists agree that the ancestors of multicellular organisms were colonial protozoa.

The biological meaning of emerging coloniality lies in its protective role from enemies and from factors of the abiotic environment.

Among higher multicellular organisms, coloniality is very pronounced in bryozoans and tunicates. However, it should be noted that columnarity is absent in many large groups of animals, for example, in mollusks, arthropods, annelids, echinoderms, vertebrates and some others, but is very strongly expressed in protozoa and coelenterates, i.e. in those groups that stand at the origins of multicellularity. Particularly interesting, of course, are those colonial organisms in which there is a morphological and functional division between individuals of the colony according to basic life functions, such as movement, nutrition, reproduction, and defense. At the lowest stage of development of coloniality are such forms as Pandorina or Eudorina among flagellates, between the members of the colony there are no differences observed. In this case, the colonial organism - an individual of the second order - has not yet subjugated individual organisms included in the colony.

With the complication of coloniality, there is an increasing subordination of the individual to the whole - an individual of the second order, and the “division of labor” between individuals goes along the lines of the functions of movement and reproduction, and subsequently nutrition. The development of coloniality is accompanied by the phenomena of polymorphism, which are also observed in the forms of colonial existence of insects. Coexistence usually leads to morphological and functional separation between individuals. With the formation of multicellularity, embryonic process-process

Before passing to some points connected with the embryonic process, it is necessary to touch upon one of the most remarkable generalizations connected with the individual development of animals, the so-called biogenetic law, or, as it is now usually called, the rule of recapitulation. This rule has penetrated deeply into modern embryology and comparative anatomy, as well as the principles of monophyly and divergent evolution. The biogenetic law is associated with the names of the two largest followers of Charles Darwin - E. Haeckel and F. Muller, although even before them some zoologists came close to this generalization (Meckel, Baer), including Darwin. Basically, this generalization is that in the ontogenetic development of organisms we find in some sequence a reflection of stages passed in the past, or, as it was sometimes said in short form, “ontogeny repeats phylogeny.”

In the evolution of animals, changes of a similar type take place with an increase in body size and with an increase in the level of organization in the evolutionary process. Thus, nematodes and arthropods lost their ciliated epithelium due to cuticularization of the integument, and the release of many animals into the air led to a reduction breathing apparatus, characteristic aquatic organisms(gills).

It is very interesting that often changes in organization accompanying a change in habitat, even in related forms, do not occur uniformly, and sometimes take diametrically opposite paths. For example, some deep sea fish a dark, almost black color is observed, while others become colorless and sometimes transparent. Some deep-sea fish have hypertrophied, sometimes stalked eyes, while others have complete reduction of the visual organs. When leaving the aquatic environment in the air, most animals developed lever organs of movement - legs (arthropods, vertebrates), however, centipedes and snakes have a long body and move by bending the body, which is more often characteristic of aquatic organisms.

All these countless cases of organ reduction cannot be considered as processes of regression in general. It would be more correct to consider them as forms of developing highly specialized organizational features as an adaptation to abnormal conditions of existence and to see in them a clear expression of the enormous ability of living beings to adapt to the most different conditions existence and to expand its living area. Thus, all this should be considered as forms of adaptation, and not as phenomena of general regression.

Returning to the general tendency of the evolutionary process - the transition from less complex, but more diverse structures to few (sometimes just one), but highly and diversely specialized, let us dwell on a number of examples. A good example would be the evolution of movement forms. The simplest have very various forms movements. Sarcodes (especially rhizomes) have a pseudopodial movement, which is carried out according to the hydraulic principle - the endoplasm rushes to one or another part of the periphery and “pulls” an outgrowth - the pseudopodia - in a more dense and elastic ectoplasm.

The movement of flagellates is ensured by the helical beating of flagella, and that of ciliates by numerous cilia. In contrast to flagella, cilia beat in the same plane, but they themselves serve, in addition to swimming, different purposes: the cilia surrounding the mouth opening create a complex flow of water that drives food particles into the pharynx. The cilia can stick together into “tassels” - cirri and on the ventral side of the gastrociliary ciliates they imitate limbs. On these cirri, ciliates can quickly “run” along the substrate. Connecting in longitudinal rows, the cilia turn into membranes capable of wave-like vibrations. In trypanoses there are also membranes along the entire body, the base of the tourniquet runs along their edge and the membrane is always in a state of wave oscillations. Gregarines move in a reactive manner, pushing off the mucus flowing from the rear end of the body. The simplest are also characterized by many different shapes hovering in water using radially diverging skeletal spines or pseudopodia.

Ciliary movement is also characteristic of countless larvae of bottom animals (worms, mollusks, echinoderms, etc.) and the smallest multicellular animals, similar in size to protozoa (rotifers and some turbellarians).

Ciliary and flagellar movements have one more feature - organisms moving in this way, when swimming, perform a rotational movement around their own axis (except for turbellarians) and, in addition, do not move rectilinearly, but along a helical line. In this way, these microscopic creatures, which have a specific gravity close to water, carry out double motor penetration into the aquatic environment. This method of movement in water is very effective. The relative speed of ciliates (the ratio of the speed of the path traveled per second to the length of the body), transferred to the size of the human body, would be the speed of a sprinter.

Thus, in protozoa we observe all sorts of methods of movement, except for flight. In coelenterates, only jellyfish, siphonophores and ctenophores have the ability to move freely. Ctenophores move with the help of the ciliated epithelium, but their movement is extremely slow. Ciliary movement is effective only for small body sizes, measured in fractions of a millimeter. With increasing body size, ciliary and flagellar movements become ineffective, since the volume of the body increases much faster (in a cube) than its surface (in a square). Subsequently, the worms developed a new form of movement - bending the body, and in connection with this they developed powerful motor muscles. In coelenterates, the muscles were completely insufficient to develop a new form of movement. 95% of coelenterates lead a motionless existence, defending themselves from enemies with a powerful skeleton and stinging apparatus. This affected the sponges, which were completely devoid of muscles, even more clearly.

The movement of jellyfish and siphonophores is accomplished in a reactive manner - the bell of the jellyfish contracts, pushes water out from under itself, and the jellyfish receives a reactive motor push with the upper side of the bell forward. For such a movement, a small motor muscle is sufficient, which is barely 1 - 2% of the body volume. Only the worms begin to accumulate motor muscles in the form of a skin-muscular sac. In nemerteans and higher worms it reaches its greatest development.

In coelenterates, a muscle layer is already created between the ectoderm and endoderm (initially in a very primitive form) in the form of layers of contractile processes of epithelial muscle cells, forming two mutually perpendicular fibers - ectoderm cells form a system of fibers longitudinal along the axis of the body, and endoderm cells - circular ones. This system of fibers seems to imitate the skin-muscular sac of worms, but does not carry out bending movements, but only performs contractile ones - along the main axis of the body. The skin-muscular sac of worms consists of two main layers of muscles - circular and longitudinal. There are also other muscles: the body of worms is, as it were, filled with muscles, their number in some worms reaches 60-70% of the total body volume (nemerteans, leeches). A huge number of lower and higher worms move by bending their bodies. Thus, within the group of coelenterates, along with the development of a bilaterally symmetrical body plan, muscular forms of animals were formed that developed a bending method of movement. These were the ancestors of turbellarians, using more powerful muscles to bend the body. Although the bending movement dominates in worms, along with it there are some other forms of movement.

Very similar processes that led to the same bending movement also took place in the evolution of deuterostomes.

The mass of lower chordates and related forms, which led either a sedentary or motionless existence, gave rise to fish-like animals with their characteristic bending movement and powerful trunk muscles. For a long period of evolution of vertebrates, the main form of their movement was bending.

Subsequently, the complication of the bending form of movement occurred with the development of segmental folds - parapodia, equipped with setae (setae). This was an additional organ of movement, so to speak, fraught with consequences, since later the limbs of arthropods were formed from them. The polymeric structure of the ringlets accordingly gave rise to forms with a large number of legs (crustaceans, arachnids).

With the formation of lever limbs, with a few exceptions (centipedes and snakes), all motor function was transferred to the leg appendages. The path of limb formation in vertebrates was somewhat different compared to that of arthropods. In arthropods, limbs were formed already in the aquatic environment (crustaceans), and in vertebrates - in the process of emerging into the air. It is remarkable that in the ancestors of vertebrates, fins also formed from folds on the sides of the body, but only in the form of two girdles - shoulder and pelvic - with two pairs of limbs. It is equally interesting that initially the fins of the ancestors of arthropods and vertebrates did not play the role of the main organs of movement, but only auxiliary ones, but it is important that in both cases the lever limbs, with all their extreme functional complexity (flight), are the only apparatus of movement. The variety of forms of movement was replaced by one form, but extremely complicated and differentiated.

Most organ systems and functions have gone through a similar evolutionary path. Let us dwell on just one more, for example, the reproduction function. It is difficult to find another function in animals that would provide such exceptional variety. First of all, of course, it should be pointed out that asexual reproduction is common in lower groups of animals, sometimes completely replacing sexual reproduction, as, for example, in many orders of flagellates and some sarcodidae. At the same time, the forms of asexual reproduction are infinitely diverse: from the creation of huge colonies of coelenterates or bryozoans, with additional phenomena of polymorphism, to the correct alternation of gamogony (sexual) and agamogony (asexual) in foraminifera and sporozoans. Asexual reproduction in protozoa is in the nature of division and budding, simple and multiple , even complex forms of schizogony. The sexual process itself has a dual form - copulation and conjugation, and in addition, parthenogenesis and fertilization. The forms of alternation of sexual and asexual development are just as diverse.

With increasing organization, the range and diversity of forms of asexual reproduction have decreased; among protostomes it is found even in annelids, and among deuterostomes - only in tunicates. In the end, asexual reproduction gives way to sexuality.

Parthenogenesis, as an exception, occurs in all groups of arthropods and vertebrates, and there are species of insects and reptiles in which males are completely unknown and reproduction occurs only parthenogenetically.

In coelenterates, a nervous system and sensory organs (vision and balance) appear for the first time. Nerve cells are genetically related not only to the ectoderm, as in all other animals except echinoderms, but also to the endoderm. The arrangement of body parts and organs in coelenterates is subject to radial-axial symmetry. Symmetry is a certain geometric order in the arrangement of similar parts of the body.

The elements of symmetry are a point (center), a line (axis) and a plane. An excellent example of radial symmetry is provided by radiolarians (Fig. 3). Similar parts of the body are located around the center of symmetry in the radial direction. Radial-radial symmetry is characteristic of organisms suspended in water and having the same environment on all sides, due to which the organism’s reaction is “the same in all directions.” Radial ray symmetry the best way corresponds to the biology of radiolarians. We also find radial symmetry in colonial phytomonads (Volvox, Eudorina, Pandorina, etc.) and some multicellular colonies, for example, in the colonial rotifer Conochilus.

However, the radial symmetry of some protozoa is not the most primitive form of body structure. Equally, planktonic existence cannot be considered the most primitive biological form. The most simply organized forms of sarcodidae (Atoelina) have an asymmetrical structure, and, apparently, it corresponds to primitive forms of organization and behavior (pseudopodial form of movement and nutrition). In addition, one can think that all pelagic forms of existence are secondary derivatives of benthic ones. An asymmetrical structure is characteristic of both ciliates and flagellates. In particular, radiolarians have an extraordinary wealth of symmetry plans for their skeleton - radial-axial, both homopolar and heteropolar, bilateral, bilateral, with the usual deviation of all these types of symmetry into asymmetry. It should be noted that in the overwhelming majority of cases, different forms of symmetry relate only to the skeleton; as for protoplasm, it, as a rule, has an asymmetric arrangement of inclusions (nucleus, pulsating and digestive vacuoles and other inclusions).

Coelenterates, both sessile and pelagic (jellyfish), are characterized by radial axial symmetry, in which similar parts are located around the axis of rotation, and this symmetry can be of a very different order depending on the angle at which the animal’s body should be rotated in order to the position coincided with the original one. Thus, 4-, 6-, 8-ray symmetry and more can be obtained, up to symmetry of the order of infinity. Radiolarians have radial-axial symmetry with identical poles, or, as they say, homopolar. In coelenterates, there is heteropolar axial symmetry: one pole of symmetry bears the mouth and tentacles (oral), the other (aboral) serves for attachment (polyp stage), or in swimming forms it carries a sensory organ (ctenophores), or is not armed with anything (jellyfish). Some jellyfish develop a stalk on this aboral side for attachment to underwater objects (Lucernariida). Violation of radial-axial symmetry occurs when the number of tentacles decreases or the shape of the oral fissure, esophagus and branches changes digestive system. The number of tentacles can be reduced to one (Mopobrachium), and then their radial arrangement is replaced by a bilateral one. The pharynx can be flattened, and then bilateral symmetry also results; this is also facilitated by the formation of siphonoglyphs in the pharynx (a groove along the pharynx).

The greatest complication of radial-axial symmetry is observed in ctenophores, where, in addition to 8-ray symmetry, in the arrangement individual parts body and organs there is 4-ray and bilateral symmetry. This is a very significant point, since most zoologists derive both stems of higher animals, both protostomes and deuterostomes, from ctenophore-like ancestors.

Heteropolar radial-axial symmetry is fully consistent with the lifestyle of coelenterates - a motionless existence in an attached position or slow swimming using jet propulsion.

On the other hand, from complex type radial-axial symmetry of ctenophores, one can move on to bilateral symmetry, or, as they say, mirror image symmetry, the only plane of symmetry of three-layered animals, symmetry fast movement, with the development of the anterior end of the body, with the central brain cluster and the main sensory organs, the dorsal and abdominal, right and left sides of the body. However, we do not know living or fossil witnesses to this transition. Here you can only use more or less reliable hypotheses.

In 1880, the famous embryologist A. Kovalevsky discovered a peculiar organism - a crawling ctenophore, which he named Coeloplana metschnikowi. By the generic name, Kovalevsky wanted to show that this organism combines the characteristics of coelenterates and planarians, i.e., flatworms. In 1886, another Russian zoologist, A. Korotnev, working on the island. Java, discovered another similar form, which he named Ctenoplana kowalewskii, also indicating in the name the combination of features of ctenophora and planarians in this organism.

Currently in the seas South-East Asia A number of similar forms have been described, united in the group Platyctenidae (flat ctenophores), but studying them has shown that it is not among them that we should look for the ancestors of flatworms, that these are simply crawling ctenophores without the ancestral organizational features of flatworms. This question has to be resolved in a different way. There are two options. According to one assumption, the ctenophore-like ancestors were originally oriented with the oral pole toward the bottom and the aboral pole upward. They then experienced flattening along the main axis of the body and the convergence of the oral pole with the aboral pole. Subsequently, the aboral sensory organ, the rudiment of the brain cluster, should have shifted to that part of the flattened body that became anterior in the direction of movement. In this way, the dorsal and ventral surfaces were developed, and the oral opening, like in many turbellarians, remained in the middle part of the ventral surface. However, the assumption of such a way of forming the body of flatworms must give way to another. It is much more likely to assume that the ctenophore-like ancestors of worms oriented sideways towards the bottom; in this case, the side of their body that was anterior in movement was immediately formed, and the mouth opening should have shifted somewhat forward along the ventral side. This assumption is more consistent with the location of the turbellarian nervous system.

As for the body structure of turbellarians, they retain a number of features of the radial symmetry of their ancestors, especially in the structure of the nervous system. They also retain ciliated epithelium on the surface of the body, the location of the mouth on the ventral side, and a number of other features borrowed from coelenterate ancestors. Until lever limbs were formed, the main mechanism of movement remained the bending movement (Fig. 4). This type of movement is possible with sufficiently powerful muscles and a certain arrangement of them in “layers” along the entire body. Both of these conditions are combined in the skin-muscle sac of worms. In this case, the motor muscles make up about half of the total volume of the body, and sometimes (nemerteans, leeches) and much more.

With the formation of limbs, the skin-muscular sac breaks up into individual muscles. The morphological basis of the motor function of the skin-muscle sac is the arrangement of contractile fibers in a mutually perpendicular direction. These are layers of circular and longitudinal muscles. Even in gregarines, myofibrils form a system of longitudinal and transverse filaments. The processes of epithelial-muscle cells of lower coelenterates also form a layer of longitudinal (from the ectoderm) and a layer of circular contractile fibers. However, in coelenterates the amount of muscle is small, the skin-muscular sac is not formed and movement is carried out in a reactive manner - only ctenophores retain ciliary movement, which, however, gives large sizes ctenophore has a very weak effect. Bending, wave-like movement is a very expedient form of movement in an aquatic environment, however, in the dense environment of soils, especially marine ones, this form of movement is not effective: animals experience hydraulic rectilinear movement. In this case, a large body cavity is formed, filled with cavity fluid. The amount of muscle in the skin-muscle sac decreases, but it is sufficient to ensure the compression of the passage in the ground by contracting the circular muscles of the body and pumping the cavity fluid forward, and then, expanding the front end of the body and wedging it in the passage, pull up the rear part of the body by contracting the longitudinal muscles.

Fundamentally the same method of movement is characteristic of bivalve mollusks, which make a passage in the ground with a wedge-shaped leg, capable of expanding when pumping cavity fluid into its lacunae, followed by pulling up the body and the shell that covers it. It is interesting to note that excellent swimmers - cephalopods - have mastered the jet movement and do not have a bending movement of the body. In their ancestors, the skin-muscle sac had already disintegrated (as in other mollusks) and the basis for creating a bending movement was lost.

In the aquatic environment, both arthropods and their characteristic movement with the help of lever-like limbs arose, but before talking about the form of movement characteristic of them, we should dwell on metamerism (segmentation) and its origin.

,

Metamerism is sometimes considered as a peculiar type of symmetry. With homonomic segmentation, as well as with other types of symmetry, there is a repetition of similar parts of the body - metameres (or somites) with the same location of the reproductive system, excretory organs, branches of the nervous (neurosomite) and circulatory (angiosomite) systems, with two parapodia each on the sides of the body, with transverse partitions - dissepiments, separating the segments from each other, with separate areas of longitudinal and circular muscles (myosomite). Only in this case, similar parts - metameres - are located not around a point or line and not on both sides of the plane, but in a linear direction along the main axis of the body. The metameric structure appeared along with the development of the secondary body cavity and the circulatory system (Fig. 5, 6). Lower worms - scolecids - lack true metamerism (Amera), it appears only in higher worms - annelids - and appears either in a small number of segments (Oligomera - bryozoans, brachiopods and all deuterostomes), or in a large number (Polymera - annelids and arthropods).

Among the main stages in the development of the animal world, along with those discussed above - the emergence of multicellularity, radial types of symmetry, the emergence of bilateral (bilateral) symmetry - the development of metamerism occupies an important place. Along with them, we can also put only the formation of lever limbs as the main organ of movement and exit from the aquatic habitat into the air.

The limbs of arthropods and vertebrates arose completely independently, although they had striking biological, evolutionary and functional similarities with pronounced differences as a consequence of differences in the nature of the skeleton of both. In arthropods, the chitinized tubular skeleton is external both in location and origin (derivative of ectoderm); in vertebrates, the skeleton is bony and internal (mesoderm). The functional similarity of the muscles serving the limb is in the antagonistic system of flexor and extensor muscles, but in arthropods they are attached to the outgrowths of the chitinous shell, and in vertebrates - to the outgrowths of the bones.

What could have caused the development of a segmented body? It should first of all be noted that the division of the body into similar parts with their linear arrangement is observed in various organisms that are at a lower level than animals with true metamerism. Such cases (pseudometamerism) are observed in shells of foraminifera (Fig. 5), turbellarians, and nemerteans. This is reflected in the ordering of the arrangement of multiple formations and organs that are polymeric in nature.

True metamerism, which appears in annelids, is a much more general and profound phenomenon, which left its mark on a large and important stage in the evolution of polymeric animals - annelids and arthropods.

The only group among the lower, non-segmented worms, leading both benthic and pelagic existence, numerous in sea basins - nemerteans - aggressive predators, possessing an organ of attack and defense - the trunk. They move in waves, bending the body, but this movement is very weak. Their skin secretes a huge amount of mucus, which may have a dual purpose: it scares away enemies and speeds up movement in the water. Apart from a few more small groups of microscopic organisms leading a bottom life, this is all that we find in sea water from lower worms. Of course, nemerteans attract the most attention among them. However, they were not able to occupy a prominent position among the population of the sea.

Most of the listed groups of scolecids are characterized by a lack of ability for rapid movement and dense integument that protects the body. For the further evolution of animals it was necessary to resolve these difficulties. Some organisms genetically related to worms found protection from enemies in the formation of strong integuments (brachiopods, bryozoans, mollusks), but lost speed of movement or even became immobile, attached forms. And only polychaete ringlets received further phylogenetic development: they developed compacted but thin integuments and a segmented structure, which made it possible to easily bend the body in all directions. Obviously, the first stages of the formation of metamerism were associated with the formation of only superficial segmentation, but gradually it captured more and more deeply located parts of the body and organs. Polymerization has captured some external and internal organs- parapodia, gill processes, reproductive, nervous, circulatory and excretory systems. The internal ordering in the arrangement of organs was subordinated to the metameric structure, and as a result of the developed metamerism, a homonomic multiplicity of segment-by-segment arrangement of organ systems arose. Thus, the need to develop rapid movement and protective coverings found its expression in the metameric structure.

Very often, during the evolution of individual groups, especially among coelenterates and worms, multiple repetitions of formations and organs of the same name are observed. Such structures are called polymeric. The polymer type is especially common in the location of the gonads. There are four of them in hydrozoa and scyphozoa, many in antheozoa, 8 male and 8 female glands in hermaphrodite ctenophora, a lot in tapeworms (cestodes), and a lot in polychaetes.

A number of polymer organs, in addition to gonads with a segmental arrangement, arose during the formation of metameric rings - parapodia, gills, excretory organs, ring nerve cords and blood vessels, ganglion clusters in the abdominal nervous system, and much more. The arrangement of many organs in echinoderms is polymeric in nature due to their pentaradial symmetry. A lot of similar examples could be given for arthropods - the number of legs in crustaceans and especially centipedes, tracheal formations in them and in insects, etc. However, in the future, a process of decreasing the number of formations of the same name is almost always observed, or, as it is called the process of oligomerization, in the number of legs, body segments, gonads, excretory organs, respiratory organs, etc.

Homonomic segmentation began to give way to heteronomous segmentation, especially in the group of arthropods, and the integument became more and more durable (chitinized), but did not lose its lightness and did not interfere with mobility due to soft and tensile joints between the individual denser parts of the chitinous shell. Subsequently, in the evolution of both main branches of animals - protostomes and deuterostomes - a striking similarity appears in the development of lever motor appendages - limbs. In arthropods, their prototype was the parapodia of polychaetes, and in vertebrates, the fins of fish. In arthropods, limbs appeared in the aquatic environment (crustaceans), and in vertebrates only with access to the air. The evolution of parapodia and limbs in polychaetes and arthropods is very good example oligomerization and transition from homonomous to heteronomous formations. In many polychaetes, parapodia are present on all segments and sometimes on hundreds of segments they have exactly the same character (homonomic polymer). In crustaceans, and even more so in arachnids and insects, the number of limbs undergoes steadily progressing oligomerization and a heteronomous type of changes, i.e. not only the number of legs decreases, but also different limbs differ from each other in structure and function. This process represents only part of more general changes in the organization of the process of cephalization (formation of the head) and tegmatization (formation of body parts). From the anterior five to six segments of the body, the head is formed - the anterior tegma of the body, and the limbs of these segments turn into sensory organs and oral appendages. The following segments form the thoracic region (insects have 3 segments, arachnids have 4, higher crustaceans have 8, etc.).

The limbs of the thoracic region are usually used for movement. The abdominal section, formed from a varying number of segments, is either devoid of limbs (insects, arachnids), or they perform another function (sexual, respiratory, etc.). The chest and abdomen form the second and third tagmas of the body.

In terrestrial groups, the limbs perform the main motor function (except for millipedes, snakes and some other groups), and segmentation as in appearance animal, and in the internal structure disappears.

The metameric arrangement is preserved only partially and only in some systems, such as the respiratory, nervous, etc. An extreme form of suppression (disappearance) of the metameric structure can be observed in mites, in the sac-like body of which all sections are fused and do not reveal a segmented structure plan. And in other arthropods, there is a fusion of the three main parts of the body, primarily the head and chest into the cephalothorax (higher crustaceans and arachnids). At the same time, sometimes the abdominal region also experiences a reduction, and then, as, for example, in crabs, the body seems to consist of one compact formation, well protected by a strong shell from enemies, and its good mobility is ensured by five pairs of limbs.

Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron

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