The complication of the structure of animal organisms occurred throughout the entire period of development of the animal world on Earth. Let's turn over the pages of this story again and mark the most important milestones in the evolution of animals (Fig. 57.4).
The first animals on Earth were also the most ancient prokaryotes. Later, protozoa appeared, from which modern unicellular organisms originate (ciliates, Sarcoflagellates, etc.). Colonial forms of ancient unicellular organisms gave rise to the first multicellular organisms, consisting of specialized cells.
The next stage in evolution was the appearance of three-layered animals similar to planaria. Unlike the two-layer ones, these new forms of animals had organ systems: digestive, circulatory, nervous, excretory, reproductive, and muscular. The nervous system evolved from a diffuse one in two-layer animals to a stem system in three-layer animals.
The appearance of a body cavity - first primary, and then secondary - next important stage in animal evolution. Ancient annelids with their closed circulatory system and abdominal nerve cord opened a new page in the history of the development of the animal world. Then, according to scientists, events developed in two directions: the evolution of arthropods and mollusks began from some annelids, and the evolution of chordates from others. Evolving in these directions, animals “acquired” respiratory organs. The structure of their nervous systems has also become more complex, and, consequently, the way of life of these animals. Material from the site
The next milestone in evolution was the exploration of land by animals. Animals developed tracheas and lungs - organs adapted to gas exchange in atmospheric air, as well as the second circle of blood circulation. Adaptations to life on land were changes in the reproduction and development of animals: internal fertilization, the appearance of eggs covered with a durable shell in reptiles and birds, intrauterine development of embryos in mammals, care for offspring. Life on land requires adaptations that protect the body from excessive loss of water, so the coverings of animals have changed. The organs of the musculoskeletal system have also undergone changes: limbs have appeared that allow them to run, jump, and fly. The nervous system and forms of animal behavior have become more complex. Warm-blooded animals and birds have colonized different parts of the planet.
On this page there is material on the following topics:
Report on the structure of animals
Position taking into account the complexity of the structure
1. habitat and structure of the body
Complication of the structure of animals in the process of evolution
How did the progressive complication of the animal world occur?
Questions about this material:
Characteristic Features general plan The structures of chordates are: deuterostome, location of the digestive system under the axial skeleton (chord or spine)(Fig. 134). Gill slits in the pharynx persist throughout life or at one of the stages of embryonic development, neural tube always located above the chord, heart - on the ventral side and pumps blood through the abdominal vessel to the head.
Chordata are deuterostome animals in which the anus is formed from the primary mouth, and the mouth is formed secondarily at the other end of the body. In addition to chordates, deuterostomes include echinoderms (starfish and others) and hemichordates.
Currently, about 50,000 species of these animals are known, living both in the aquatic environment and on land. Phylum Chordata includes:
Subphylum Larvalchordates(Urochordata) , subtype Skullless(Acrania): class Cephalochordates; P phylum Vertebrates(Vertebrata): class Cyclostomes, class Cartilaginous fishes, class Bony fishes, class Amphibians, class Reptiles, class Birds, class Mammals.
Veils. The skin is represented by the epidermis and dermis. The epidermis can be represented by single-layer and multilayer epithelium, the dermis is fibrous connective tissue. Scales, feathers, hair, nails, claws and other horny formations are derivatives of the epidermis. Various glands are formed in the skin: those that form mucus, sebaceous, sweat, and odorous.
© Musculoskeletal system . The skeleton is internal, represented by a notochord; in vertebrates, the notochord is replaced by the spine. Vertebrates are characterized by the development of two pairs of limbs. The muscular system is represented by smooth and striated muscles.
© Digestive system. In cephalochordates, in the form of a straight tube, the digestive glands are poorly developed. In vertebrates, glands lying outside the digestive tract - the pancreas and liver - develop well. The alimentary canal is differentiated into the oral cavity, pharynx, esophagus, stomach and intestines.
© Respiratory system formed by gills in lower chordates, lungs in adult amphibians and terrestrial vertebrates, part of the gas exchange in chordates occurs through the skin.
© Circulatory system closed. In cephalochordates there is no heart; in the rest, due to an increase in the intensity of metabolism, the heart appears and becomes more complex.
© Excretory system. Lancelets have nephridia, while other chordates have kidneys, ureters and a bladder.
© Nervous system divided into central and peripheral. In vertebrates, due to an active lifestyle, the anterior part of the neural tube turns into the brain, the sense organs become more complex, and the spinal cord is formed. The peripheral nervous system is represented by nerves extending from the central nervous system.
© Reproductive system. Gonads - testes in males and ovaries in females, excretory ducts - oviducts and vas deferens. Most chordates are dioecious animals
© The appearance of chordates was accompanied by the most important aromorphoses: 1. The axial skeleton, the notochord, appeared. The notochord is an elastic cord of cartilaginous tissue formed from endoderm and surrounded by a connective tissue membrane. In lower chordates, the notochord remains throughout life, while in higher chordates it is replaced by a spine. 2. The central nervous system takes the form of a tube with a canal (neurocoel) inside and is located on the dorsal side of the body. The neurocoel is formed as a result of the fact that the neural plate, which is laid down in the ectoderm, curls into a tube. The anterior part of the neural tube in vertebrates becomes more complex and becomes the brain. This tubular structure of the nervous system promotes metabolism not only from the surface, but also from the inside, which makes it possible to increase the mass of the brain. 3. Feature chordates is manifested in the fact that the walls of the pharynx are penetrated by gill slits, which ensures active gas exchange when pumping water with the mouthparts through the gill slits.
Lancelet
The only class Cephalochordates belongs to the subtype Cephalochordates, which includes only about 30 species of marine animals living in shallow waters. A typical representative is the lancelet (Branchiostoma lanceolatum), the size of which reaches 8 cm. The body of the lancelet is oval, narrowed towards the tail, compressed laterally. On the back of the body there is a caudal fin in the shape of a lancet - an ancient surgical instrument. There are no paired fins; there is a weakly defined dorsal fin. On the sides of the body from the ventral side hang two folds, which grow together on the ventral side and form peribranchial a cavity communicating with the pharyngeal slits and opening with an opening to the outside.
© Veils.Represented by skin consisting of a single layer of epidermis and a thin layer of dermis.
© Musculoskeletal system. The chord extends along the entire body, becoming thinner in the front and back parts of the body. The notochord extends further into the anterior part of the body than the neural tube, hence the name of the only class - cephalochordates. The notochord is enclosed in a connective tissue case, which forms supporting elements for the dorsal fin and divides the muscle layers into segments using connective tissue layers. The muscles are formed by striated muscles.
© Digestive system. On the front of the body there is a mouth opening surrounded by tentacles (up to 20 pairs). Oral opening leads into the large pharynx, the filtering apparatus. Through the cracks in the pharynx, water enters the atrial cavity, food particles are captured by the ciliated epithelium and directed to the bottom of the pharynx, where the endostyle is located - a groove with ciliated epithelium, which drives mucus forward, then along the dorsal groove - to the intestine. There is no stomach; there is a hepatic outgrowth homologous to the liver of vertebrates. The intestine does not make loops and opens with the anus at the caudal fin. Digestion of food occurs in the intestines and in the hollow hepatic outgrowth, which is directed towards the head end of the body. Interestingly, the lancelet has preserved intracellular digestion; intestinal cells capture food particles and digest them in their digestive vacuoles. This method of digestion does not exist in vertebrates.
© Respiratory system. The pharynx has more than 100 pairs of gill slits leading into the peribranchial cavity. The walls of the gill slits have blood vessels in which gas exchange occurs. With the help of the ciliated epithelium of the pharynx, water is pumped through the gill slits into the peribranchial cavity and through the opening (atriopore) it is discharged out. In addition, the skin also takes part in gas exchange.
© Circulatory system. The blood of the lancelet is colorless and does not contain respiratory pigments. The transport of gases occurs as a result of their dissolution in the blood plasma. The circulatory system is closed, one circle of blood circulation. There is no heart, and the blood moves thanks to the pulsation of the gill arteries (Fig. 135), which pump blood through the vessels in the gill slits. Arterial blood enters the dorsal aorta, from which blood flows through the carotid arteries to the anterior part, and through the azygos dorsal aorta to the posterior part of the body. Then, through the veins, the blood returns to the venous sinus and is directed through the abdominal aorta to the gills. All blood from the digestive system enters the hepatic process, then into the venous sinus. The hepatic outgrowth, like the liver, neutralizes toxic substances that enter the blood from the intestines, and, in addition, performs other functions of the liver. Such a structure circulatory system not fundamentally different from the circulatory system 
vertebrates and is often considered as its “prototype”.
© Excretory system.
The excretory organs of the lancelet are called nephridia and resemble the excretory organs of flatworms - protonephridia. Numerous nephridia (about one hundred pairs, one for two gill slits), located in the pharynx, are tubes that open with one opening into the coelom cavity, the other into the peribranchial cavity (Fig. 136).
On the walls of the nephridium there are club-shaped cells - solenocytes, each of which has a narrow canal with a ciliated hair. Due to the beating of these hairs, fluid with metabolic products is removed from the nephridium cavity into the peribranchial cavity.
© central nervous system formed by a neural tube with a cavity inside. The lancelet does not have a pronounced brain. In the walls of the neural tube, along its axis, there are light-sensitive organs - the eyes of Hesse. Each of them consists of two cells - photosensitive and pigment, they are able to perceive the intensity of light. The organ of smell is adjacent to the expanded part of the neural tube.
© Reproduction and development. Lancelets are dioecious; the gonads (gonads, up to 26 pairs) are located in the body cavity in the pharynx. Reproductive products are excreted into the peribranchial cavity through temporarily formed reproductive ducts. Fertilization is external, the zygote undergoes fragmentation and turns into a morula, blastula, gastrula, and neurula according to the classical scheme. There is a larval stage. The larva actively moves with the help of cilia covering the entire body, then - due to the lateral bends of the body. The larva leads a pelagic lifestyle for up to three months, then switches to life on the bottom.
The structural features and features of embryonic development characteristic of chordates were studied by the Russian scientist A. O. Kovalevsky. But there are no sufficient grounds to consider lancelets as the direct ancestors of vertebrates. They developed along the path of adaptation to a benthic lifestyle with a filter feeding type.
Skullless animals retained a number of characteristics of their invertebrate ancestors:
¨ excretory system nephridial type;
¨ absence of differentiated sections in the digestive system and preservation of intracellular digestion;
¨ filtering method of feeding with the formation of a circumbranchial cavity to protect the gill slits from clogging;
¨ metamerism (repeated arrangement) of the genital organs and nephridia;
¨ absence of a heart in the circulatory system;
¨ poor development of the epidermis; it is single-layered, like in invertebrate animals.
Subphylum Vertebrata
Superclass Pisces
Chordates are the most highly organized creatures of all representatives of the animal kingdom. Character traits structures allowed them to become the pinnacle of evolution.
Traits of chordates
The main features of these animals are the presence of a notochord, a neural tube and gill slits in the pharynx. Chordates are organisms in which the listed characteristics can vary significantly.
So, the skeleton can be external and internal. And the development of chordates in ontogenesis can be characterized by the fact that the gill slits are overgrown during the embryonic development of organisms. At the same time, they develop other respiratory organs - air sacs or lungs.
Axial skeleton
The main characteristic of chordates is the presence of a notochord. It is internal, which in the form of a solid cord passes through the entire body. Not many representatives of this type retain the notochord throughout their lives. These include different types lancelets, representing the class Cephalochordates of the subtype Invertebrates.
In other representatives, the notochord develops into a skeleton. Only in a few it consists of cartilage tissue. Amphibians, reptiles, birds and mammals have a completely ossified skeleton. In the process of evolution it becomes more complex. Its components are the skull, spine, rib cage, belts and directly the upper and lower limbs.
Gill slits in the pharynx
Chordates are animals in which the organs of the respiratory system are formed as outgrowths of the pharynx. This is their main difference from invertebrates. In this group they are derivatives of the limbs.
Of course, not all chordates have this anatomical feature. Gill slits are preserved in lancelets and sharks and rays. In animals that are adapted to breathing atmospheric oxygen, they become overgrown in the early stages of embryonic development. Afterwards the lungs are formed.
Features of the nervous system
A system that provides interconnection between the body and environment, in chordates it is initially formed as a neural tube. It is of ectodermal origin.
Chordates are highly developed animals, largely due to the structural features of the nervous system. Thus, in mammals it is represented by the spinal cord, located in the spinal canal, as well as the brain. They are parts of the central nervous system. The brain is reliably protected by the bones of the skull, which are fixedly connected. It is differentiated into departments based on functional characteristics. Anatomically, the brain is connected to the spinal cord through the opening formed by the vertebrae. The peripheral part of the system is formed by the spinal and cranial nerves. They act as a “transport highway,” uniting a complex organism into a single whole and coordinating its work.
Aquatic inhabitants are fish. Their streamlined body is covered with scales, they are adapted to gill breathing, and move with the help of fins.
The first animals to reach land were amphibians. These are frogs, toads, newts, worms and fish snakes. Their common name due to the fact that they live on land, breathe with the help of their lungs and skin, but the process of their reproduction occurs in water. Like fish, their females lay eggs in the water, which the males water with seminal fluid.
Typically terrestrial animals are reptiles. Lizards, snakes, turtles and crocodiles spend only time hunting in the water. They reproduce by eggs, which they lay in special shelters on land. Their skin is dry and covered with dense scales.
The last trait was inherited from reptiles by birds. The unfeathered part of their legs is called the tarsus. It is she who is covered with small scales. Scientists consider this fact as evidence of origin in the process of evolution. Birds are capable of flight due to many external and internal structure. These are modified forelimbs, feather cover, a light skeleton, the presence of a keel - a flat bone to which the muscles that move the wings are attached.
Finally, Beasts, or Mammals, are the crown of evolution. They are viviparous and feed their young with milk.
Chordata animals are the most complexly organized, diverse in structure, playing a vital role in nature and human life.
ANSWER: Light and muscular fins.
What do the numerous teeth on the jaws of the first bird, Archeopteryx, indicate?
ANSWER: On the origin of birds from reptiles.
What does the reproduction of some mammals (echidna and platypus) with eggs indicate?
ANSWER: On the origin of mammals from reptiles.
At what stage of development does the Colorado potato beetle harm plants?
ANSWER: At the larval and adult stages.
What signs are characteristic of coelenterates?
ANSWER: Two layers of cells, intestinal cavity, radial symmetry, stinging cells, tentacles, predators, high ability to regenerate.
What is the role conditioned reflexes in the life of animals?
ANSWER: Adapt to specific living conditions.
What are the convergent similarities between a crocodile, a frog and a hippopotamus?
ANSWER: In a similar arrangement of eyes and nostrils on the head (on a hill), which arose due to living in similar conditions.
Why can the number of commercial herbivorous fish sharply decrease when predatory fish are destroyed in a reservoir?
ANSWER: The destruction of predators will first lead to an increase in the number of herbivorous fish, but then their density increases, competition intensifies, and diseases quickly spread, which leads to a reduction in their numbers.
Why doesn't the circulatory system of insects perform the function of transporting gases?
ANSWER: Because their respiratory organs - tiny branched tubes - tracheas - deliver oxygen directly to the cells, and carbon dioxide enters and is removed from the cells into the trachea.
A rabbit egg is 3000 times smaller than a frog egg and contains few nutrients. Why doesn't a rabbit embryo die from lack of nutrients?
ANSWER: The rabbit embryo develops in the uterus and is supplied with nutrients from the mother's blood.
Why do insectivorous birds arrive in spring in central Russia much later than herbivorous birds?
ANSWER: Insectivorous birds cannot provide themselves and their chicks with food due to the lack of insects.
How do birds adapt to unfavorable winter conditions in central Russia?
ANSWER: Birds develop thick feather cover, store fat, some store food or switch to other types of food, some migrate, fly, and roam.
What adaptations do mammals have to seasonal environmental changes?
ANSWER: They hibernate (bears, chipmunks), migrate to other territories (whales, saigas), store food and accumulate fat, molt and grow thick fur.
What is the complexity of the structure of the circulatory system in amphibians compared to fish?
ANSWER: The heart becomes three-chambered, two circles of blood circulation are formed. In a small circle, blood from the ventricle enters the lungs, is saturated with oxygen and enters the left atrium. In a large circle, blood from the ventricle is directed to the organs of the body, and from there to the right atrium. The body receives mixed blood.
Why do turtles hibernate with the onset of a hot and dry period in the desert and endure unfavorable conditions in this state?
ANSWER: In a state of rest (anabiosis), metabolism slows down, the need for food and water, which is not enough during the hot and dry period, is sharply reduced.
What structural features were formed in chordates during the process of evolution?
ANSWER: An internal axial skeleton appeared - a chord or spine, the nervous system in the form of a tube was located on the dorsal side of the body, gill slits appeared in the digestive tube
Explain why earthworms avoid waterlogged areas of soil and crawl to its surface.
ANSWER: Earthworms breathe over the entire surface of their body, and in waterlogged areas there is little oxygen in the soil.
Many birds live in pairs during the breeding season and form flocks in winter. Explain why birds temporarily unite in flocks.
ANSWER: Many forms of behavior in birds are determined by instincts - complexes of unconditioned reflexes preserved by natural selection. In the spring, the birds begin to show their reproductive instinct, so they form pairs. In winter, they also form flocks based on instinct, which helps them find food more easily, protect themselves from enemies, etc.
What features are characteristic of mollusks?
ANSWER: The body is soft, not segmented, most have shells, have a mantle and mantle cavity, and the circulatory system is not closed.
What are the similarities between flatworms, roundworms and annelids?
ANSWER: The presence of a skin-muscular sac, digestive, excretory, nervous, reproductive systems, the head part is secreted in the body, reproduction with the help of eggs.