Bisexual chordates. Type Chordata. General characteristics. Structural features

Lecture: chordate animals. Characteristics of the main classes

chordate animals

Evolutionarily, this taxon is the youngest. Today there are 60 thousand species.

Distinctive features of their structure and metabolism are:

1. The presence of an elastic rod-chord or a full-fledged spine;

2. The neural tube is located on the dorsal side of the body, forming the spinal cord;

3. Developed brain;

4. Oxygen respiration;

5. The intestine does not enter the tail section.

Among the chordates there are marine inhabitants, as well as species living on land, in the soil, in the air.

lancelets

These are marine animals of a primitive structure. Many scientists consider them as a transitional group from invertebrates to vertebrates.

The structural features of this animal are:

thin chord;

the presence of gills;

located in the dorsal part of the body neural tube.

The lancelet has a translucent body, on the back and ventral side, as well as the end of the tail, there are fins, the main function of which is movement and maintaining balance in the water. The skin is thin, one-layer. Central nervous system represented by a single tube. There is no differentiation between spinal cord and brain. The tentacles located around the mouth are used as sensory organs. There are simply arranged light-sensitive eyes. The skeleton is represented by a chord, along which are strands of muscles.

The pharynx can occupy up to a third of the length of the body, and gill slits are also located in it. The intestine is primitive, undifferentiated, ending in an anus. There is a hepatic outgrowth that secretes enzymes. The excretory system is similar to that of annelids - a system of convoluted tubules, metabolic products are excreted through the gills. The lancelet breathes the entire surface of the body. The circulatory system is developed, partially closed, there are vessels. Lancelets are dioecious, fertilization is external. The eggs develop in an aquatic environment.

Fishes

A characteristic feature is the presence of developed jaws and gill breathing. The outer covers are leather. In bony it is covered with scales, in cartilaginous it is naked, with rows of plaques. They can move with the help of fins.

The spinal cord is located inside the spine. There are ribs. The brain consists of five sections. Along the body is the organ of tactile senses - the lateral line. Eyes of complex structure. There are organs of taste and smell. The organs of hearing are paired. Most representatives have a single circle of blood circulation and a two-chambered heart.

Fish grab food with their mouths, tear them apart and hold them with their teeth. The lump enters the throat, then through the esophagus into the stomach. The anal opening is located in the region of the posterior pelvic fins. The body of the fish maintains its osmotic pressure by excreting excess salts by the kidneys.

Fish are bisexual for the most part, hermaphrodites are rare, some of them are capable of self-fertilization. Mating behavior is quite complex - nests are built where eggs are laid. Fertilization is external.

Amphibians

This is a relatively small taxon of animals adapted to living on land and in water. For reproduction, most species require a reservoir, since in the larval stage the amphibian lives in the aquatic environment.

The skin is thin and smooth, it is easily permeable to gases and liquids, and is equipped with a huge number of capillaries for additional gas exchange. It contains many mucus-producing glands. The skeleton has a complex structure, divided into the spine, skull and limbs. The muscle system has a rather complex structure, providing mobility. Respiratory organs are gills, lungs and skin.

The circulatory system consists of two circles of blood circulation, the heart is three-chambered (two atria and one ventricle). The metabolic rate is low, the animals are completely dependent on the external temperature, they are cold-blooded. All amphibians are predators. The trapping organ is a long tongue. The intestines are divided into sections and open into the cloaca. The excretory system is represented by the kidneys and bladder having an outlet to the cloaca. Some substances are removed through the skin.

The brain is larger than that of fish, there are nerve plexuses in the area of the shoulders and lower back. The eyes are adapted to the atmospheric environment, equipped with eyelids. There are two olfactory sacs. Hearing aid pretty complex structure, there is a tympanic membrane and a middle ear. The skin contains many nerve endings and is the organ of touch.

All species have separate sexes, fertilization is external. The development is complex, stage by stage - egg, larva, metamorphosis, adult. Eggs are laid in water by most species.

reptiles

This taxon contains mainly terrestrial species, distinctive features which is:

dry skin covered with scales or scutes;

more pronounced division of the spine into sections. The head can turn;

all but snakes have a chest, there are intercostal muscles that provide breathing.

The developed nervous system, the brain consists of 5 departments. There are 6 sense organs. Breathing is pulmonary. The heart is three-chambered, a partial septum appears in it, separating venous and arterial blood. A more complex gastrointestinal tract, where differentiation into departments is more pronounced. excretory system in the form of kidneys, Bladder and cloacas. Reproduction is bisexual, with internal fertilization. The embryo develops in an egg equipped with a leathery or calcareous shell. Some species have live birth. The development is direct. There is care for offspring.

Birds

This is a class of egg-laying warm-blooded animals adapted to flight in the atmosphere. In this regard, there were qualitative changes in the structure of the body:

forelimbs in the form of wings;

lightweight skeleton, the presence of a pectoral keel and powerful muscles;

the presence of a feather cover;

powerful respiratory system consisting of lungs and five pairs of air sacs;

streamlined body shape;

the presence of a beak;

four-chambered heart;

there are no teeth, food is crushed in the stomach, for which the bird swallows sand and pebbles;

fast metabolism.

A very developed cerebrum with a large cerebellum that controls body position and motor activity. Complex intellectual behavior.

Reproduction is bisexual, fertilization is internal. The embryo develops in the egg. Most species are monogamous. Marriage behavior is complex.

Birds are important contributors to ecosystems. They contribute to the spread of plants by spreading seeds, predators regulate the number of small animals.

mammals

The most evolutionarily young and progressive taxon. It has a number of fundamental differences from all other animals:

Live birth (except for the first animals) and feeding offspring with milk;

Real warm-bloodedness;

Strong development of sebaceous and sweat glands;

Division of the internal cavity of the body into two by a diaphragm;

High development of the nervous system;

differentiated teeth.

Representatives of this class are capable of higher nervous activity, an active lifestyle.

The chordate type has more than 40 thousand living species of animals. This includes non-cranial (tunicates and lancelets) and cranials (cyclostomes (lampreys), fish, amphibians, reptiles, birds and mammals). Representatives of this type live throughout the globe and in all habitats. Most chordates lead an active, mobile lifestyle, but there are species that are attached to the substrate - tunicates. The size and weight of the body vary widely in this type and depend on the species and habitat of the animal.

Despite the fact that animals united in the chordate type are very different in appearance, features internal structure, lifestyle and habitat,

they have a number common features. It will help to identify this similarity. general characteristics chordates.

All chordates have:

The axial skeleton, which is represented by a notochord in non-cranial animals and a vertebral column in cranial ones. The skeleton has the form of a strand, performs a supporting function and gives elasticity to the body.

Gill slits in the pharynx. In protostomes, which live all the time in water and do not leave it, gill slits remain throughout life. And in deuterostomes that left and then returned there again (dolphins, whales, crocodiles), and terrestrial animals, gill slits exist only at certain stages and then disappear. Instead of them, the lungs function - the organs of terrestrial respiration.

The central nervous system (CNS), which is located in the form of a tube on the back. In primitive chordates, it remains in the form of a hollow tube throughout life, and in highly organized animals it is divided into the brain and spinal cord. And the nerve endings that depart from the central nervous system form the peripheral nervous system.

The heart, like the neural tube, is located on the ventral side of the body.

Chordate animals have distinctive features within the species, which is associated with their lifestyle and habitat, as well as adaptation to it. In addition to signs of difference from other organisms, chordates also have similarities with other animals. These similarities are:

Bilateral symmetry, which is inherent in insects and other organisms.

The whole (otherwise the secondary cavity of the body), in which the internal organs are located. The secondary cavity appears in annelids.

They have a secondary mouth, which is formed at the gastrula stage by breaking through the wall.

Metameric (segmental), clearly expressed at the embryonic stage and in primitive chordates, in adult animals it can be traced in the axis of the spine. Due to this, the chordate type shows signs of similarity with and insects.

The presence of organ systems - circulatory, respiratory, nervous, digestive, excretory, sexual.

Thus, the chordate type unites animals that are characterized by bilateral symmetry and, in general, the presence of gill slits in the early stages of development and the appearance of an internal skeleton - a chord, over which the neural tube is located. Under the notochord is the digestive tube.

Type Chordates unites the most developed animals of our planet. All of them are adapted to their living conditions, but they also have common features.

Type attributes

The name of the type comes from the word "chord". This is the name of an elastic, non-segmented rod, which is the internal skeleton of primitive chordates.

In highly developed species of chordates, the notochord is present only in the embryonic period, and then it is replaced by the spine.

Other type features are:

bilateral symmetry of the body;
tubular structure of the central nervous system;
gill slits in the pharyngeal region of the digestive tube.

Bilateral symmetry of the body is also characteristic of some other types of animals.

Development of chordates

The exact origin of chordates is unknown. It is believed that their ancestors were sedentary marine animals, similar to echinoderms.

With access to land, chordates have limbs adapted for jumping and running on land.

Rice. 3. Skeletons of chordate classes.

The chordate skeleton consists of:

skulls;
spine;
limb skeleton.

Chordata mastered various habitats. Adaptation to them went along the path of development:

nervous system;
skin;
methods of reproduction;
internal organs;
sense organs.

As a result, birds and some mammals have mastered the air environment and have wings. Modern terrestrial chordates exhibit complex behavior and are capable of logical thinking.

Raven is very smart. When he was offered a drink in a jug, from which he could not get it with his beak, he began to throw objects into the jug to raise the water level.

Birds and mammals also have an important property - warm-bloodedness, due to which they are able to live in a cold climate.

Systematics

The classification of animal types is based on the characteristics of reproduction, body structure and physiology.

Table "Classes of chordates"

*Class*	*Structural features*	*Features of physiology*
	Notochord only in larvae, adults are immobile benthic animals	Passive nutrition, excretion through the surface of the body
lancelets	Notochord, gill slits in the pharynx; lack of heart and organs of hearing	Passive nutrition and breathing
Cyclostomes (lampreys)	Chord, lack of jaws; look like fish
cartilaginous fish	Cartilaginous skeleton, highly developed sense organs	gill breathing
bony fish	Skeleton, swim bladder	gill breathing
Amphibians	2 pairs of limbs adapted for movement on land	Mixed breathing: skin and lungs, in larvae - gills
reptiles	Skin covered in scales	Lung respiration only, internal fertilization
	Wings, feather cover	Difficult mating behavior, flights
mammals	Skin covered with fur	Live birth, breastfeeding

The high degree of organization of chordates does not mean that they are better adapted to life than primitive animals. Many species of the type today are endangered and exist only thanks to human protection.

What have we learned?

We have given a general description of chordates. The morphophysiological characteristics of each class has its own characteristics. The presence of a notochord, a neural tube, and slits in the pharynx are signs that unite animals into this type. Moreover, most of them have these signs only in the embryonic period.

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The type includes about 43 thousand species of animals that inhabited the seas, oceans, rivers and lakes, the surface and soil of continents and islands. Appearance and the sizes of chordates are varied, as are their sizes: from small fish and frogs 2-3 cm to giants (some species of whales reach a length of 30 m and a mass of 150 tons).

Despite the huge variety of representatives of the Chordata type, they have common organizational features:

1. The axial skeleton is represented by a chord - an elastic rod located along the dorsal side of the animal's body. Throughout life, the notochord is retained only in the lower groups of the type. In most higher chordates, it is present only at the embryonic stage of development, and in adults it is replaced by the spine.

2. The central nervous system looks like a tube, the cavity of which is filled with cerebrospinal fluid. In vertebrates, the anterior end of this tube expands in the form of bubbles and transforms into the brain, in the trunk and tail sections it is represented by the spinal cord,

3. The anterior part of the digestive tube - the pharynx - is pierced by gill slits, through which it communicates with the external environment. In terrestrial animals, slits are present only in the early period of embryonic development, while in aquatic chordates they persist throughout life.

4. The circulatory system is closed, the heart is located on the ventral side, under the chord and digestive tube.

Rice. Diagram of the structure of a chordate animal

5. Other than these distinguishing features, characteristic only of chordates, the following are inherent in them: all of them are bilaterally symmetrical, secondary cavities, deuterostomes.

6. Type Chordata is divided into three subtypes and 12 classes. Let's consider the most important of them.

Subtypes and classes of chordates

The type of chordates includes three subtypes - Cranial, Larval - chordate and Vertebrates. Chordates have an internal skeleton-chord in the early stages of development. Chordates occupy the main environments of life: water, land-air and soil. These are bilaterally symmetrical three-layered animals. Chordates include fish, amphibians, reptiles, birds, and mammals.

Subtype non-cranial

Class lancelets

Lancelets are a small group of animals reaching several centimeters in length. The reason for such a strange name was that the rear end of the body of these animals is similar to the blade of a surgical knife - a lancet. The body of the lancelet is elongated, laterally compressed, its anterior and posterior ends are pointed. The head is not expressed.

Vertebrate subtype

Cartilaginous fish class

About 660 species belong to the class of cartilaginous fish. This group includes well-known sharks (frilled, tiger shark, katran) and rays (stingray, saw-fish, manta), combined into two separate superorders, as well as whole-headed (chimeras). These are mostly large animals - the whale shark reaches a length of 20 meters. Like all vertebrates, representatives of this class are bilaterally symmetrical animals.

Class bony fish - the most numerous group of vertebrates. It has about 20,000 species belonging to 4 subclasses: ray-finned, multi-finned, cross-finned, lungfish.

We list the main representatives of the class:

detachment of sturgeons - beluga, sturgeon, sterlet;

salmon-like detachment - salmon, salmon, trout;

detachment cyprinids - bream, carp, crucian carp, silver carp;

detachment cod-like - cod, hake, pollock;

perch-like detachment - perch, horse mackerel, mackerel, pike perch.

Bony fish inhabit a wide variety of water bodies: fresh (ponds, rivers, lakes) and salty (seas, oceans). The body shape of these animals is mainly fusiform, streamlined, which allows to reduce water resistance when swimming.

Bony fish are vertebrates that have a number of adaptations to an aquatic lifestyle:

The mode of transportation is swimming;

Streamlined body shape;

Fixed articulation of the head with the body;

Tiled-like scales;

The organs of motion are fins, which, in addition, perform the function of stabilizers (ensure the stability of the body in water) and depth rudders;

Breathing with gills;

The presence of a swim bladder;

A special organ is the lateral line.

Class amphibians (amphibians)

This class unites animals, the peculiarity of which is that adults can live both on land and in water. However, their reproduction and development of eggs almost always occurs in the aquatic environment. This class includes about 3000 species, divided into three orders:

A detachment of legless amphibians, represented by a small group of organisms with reduced limbs and a tail - caecilians;

A detachment of tailed amphibians, which includes salamanders, newts, proteas, sirens;

The order of tailless amphibians, which has the greatest species diversity, includes animals such as frogs, toads, tree frogs, spadefoot, and toads.

Almost all amphibians have small size. The body of adults is divided into the head, trunk, tail (caudated in the detachment) and two pairs of limbs (in worms, the limbs and their belts are reduced). In connection with landfall, the body of the majority is flattened in the dorsal-abdominal direction, and the head is movably articulated with the body. The skin of amphibians is naked, so water and gases can freely diffuse through it.

Class reptiles or reptiles

There are about 6600 species of reptiles in the world fauna. The living reptiles are grouped into the following groups:

Detachment of the Turtle (representatives: caiman turtle, green turtle);

Order Beakheads (a very ancient group with the only surviving species - the tuatara, which is found in New Zealand. Among modern reptiles, the tuatara is closest to the scaly order;

Order Scaly (these include animals such as chameleons, lizards, snakes);

Detachment Crocodiles (representatives: Mississippi alligator, Nile crocodile, etc.).

Representatives of the reptile class are real land animals. The development of adaptations for life on land allowed the ancestors of these animals to leave aquatic environment and spread widely across the earth. However, in all orders (with the exception of beakheads) there are forms that have passed to life in water for the second time.

bird class

This class includes about 8600 species of living birds. They are subdivided into two superorders. Superorder Penguins (or Floating). Representatives of this group (king penguin, little penguin, Galapagos penguin, etc.) are large animals, they cannot fly, the main mode of transportation is swimming. The forelimbs are modified into flippers. Penguins are common in the cold regions of the southern hemisphere - in Antarctica and the islands of the Subantarctic. The superorder New palatine, or Typical birds, is represented by big amount orders: ostriches, anseriformes, chickens, cranes, bustards, waders, gulls, owls, woodpeckers, parrots, passerines, etc. Almost all the features of birds are associated with the development of airspace and the presence of adaptations for flight. The body of birds has a streamlined aerodynamic shape. It is covered with feathers, which are divided into contour and down. The axial part of the pen is the rod and the core. Quite the feather is immersed in the skin, and the fans depart from the rod. In the contour pen, they are formed by barbs of the first order, bearing barbs of the second order, fastened together by small hooks in such a way that a plate is formed. The core of the down feather is thin, there are no hooks. A downy feather, the beards of the first order of which extend in a bundle from the quill, is called down. Contour feathers give the body a characteristic shape, while down feathers serve as a heat-insulating material. Periodically, birds change their plumage - molting.

Class Mammals (or Beasts)

The class mammals is the last of the classes of vertebrates we are considering, representing the most highly organized group in the entire animal kingdom. Mammals inhabit a wide variety of habitats; they can be found in tropical forests and arctic deserts, in mountains and in ocean expanses.

This class includes seemingly completely different animals: the blue whale and the common hedgehog, the African elephant and the squirrel, the bat and the kangaroo, etc. We also systematically belong to this group. What, nevertheless, is common between all these different creatures?

Mammals are characterized by the following features:

development of hair on the skin;

a large number of skin glands: sweat, sebaceous;

the presence of mammary glands that secrete milk;

feeding the young with milk and caring for the offspring;

live birth (with the exception of single pass);

constant body temperature - homoiothermia;

intensive flow of basic life processes;

four-chambered heart, two separate circles of blood circulation;

lungs of an alveolar structure, there is an epiglottis;

the presence of a diaphragm separating the abdominal and thoracic cavities;

teeth are differentiated into incisors, canines, premolars, molars;

there are seven cervical vertebrae in most species (the exceptions are dugongs, manatees and sloths);

large relative sizes of the brain, significant development of the cerebral cortex, high level development of the sense organs.

There are about 4.5-5 thousand species of mammals in the world, belonging to three subclasses and 21 orders, although some experts distinguish only 18 orders:

Subclass I - cloacal (oviparous or first animals) with one detachment - single pass;

II subclass - marsupials with one detachment of marsupials;

Subclass III - placental (or higher animals) with nineteen orders: insectivores, bats, woolly-winged, semi-monkeys (lemurs), monkeys (primates), edentulous, lizards (pangolins), aardvarks, rodents, hares, carnivores, pinnipeds, cetaceans, artiodactyls , calluses, equine, hyraxes, proboscis (elephants), lilac (sea cows).

In a number of representatives of this class, the size and weight of the body vary within a very wide range. The smallest animal of the world fauna, the baby shrew, weighs only 1.2 g and reaches 45 mm in length, and the largest is the blue whale, about 150 tons and 33 m, respectively. The skin of animals is represented by the stratum corneum of the epidermis, the Malpighian layer, the corium (skin itself), and also a layer of connective tissue, which may contain (sometimes significant) fat accumulations. Animals of this class are characterized by a large number of horn formations, which include:

hair (characteristic of almost all mammals, except for cetaceans), as well as their various modifications: whiskers or sensitive hair (for example, "whiskers" in cats), bristles (pigs), needles (hedgehogs, porcupines, echidnas);

scales (in pangolin lizards);

horny plates (armadillos);

horns in rhinos, horn covers in bovids (cows, goats);

nails (human and other primates);

claws (predators, anteaters);

hooves (horses, cows, tapirs, hippos).

Often the hairline is highly developed and forms thick fur. There are two types of hair:

Long and relatively sparsely spaced, called ostia;

Short and dense, called undercoat.

The skin is rich in glands, among which sebaceous and sweat glands are distinguished. The sebaceous glands have a groin-shaped body, from which channels extend, opening in the hair bag. These glands secrete an oily secret. Sweat glands look like tubes folded into a ball, opening on the surface of the body. The lactiferous and odorous glands are modified sweat glands. The mammary glands that secrete the milk necessary for feeding offspring have a vine-like structure and open on the nipples. In monotremes (platypus, echidna), these glands have a tubular structure and do not open on the nipples, since they do not exist, but into hair bags. Platypus and echidna cubs simply lick drops of milk from their mother's fur. The skeleton has a number of features. The surface of the vertebrae is flat, not saddle-shaped, as in birds, and not convex-concave, as in reptiles. The spinal column is divided into five sections:

Cervical (in the vast majority of cases it consists of 7 vertebrae);

Thoracic (numbers from 9 to 24, more often 12, vertebrae);

Lumbar (2-9 vertebrae);

Sacral (from 4 to 9, while true sacral vertebrae - 2);

Tail (contains from 3 to 49 free vertebrae).

The girdle of the forelimbs (shoulder) is represented by the shoulder blades and clavicles (absent, for example, in ungulates), the coracoid is reduced and fuses with the shoulder blade, forming the coracoid process. The free forelimb consists of: the humerus, ulna and radius, wrist, metacarpal bones and phalanges of the fingers. The belt of the hind limbs (pelvic) is represented by pelvic bones(sciatic, pubic and iliac). The free hind limb consists of the femur, tibia, tibia, tarsus, metatarsal bones, and phalanges of the fingers.

The digestive system is represented by: oral cavity, pharynx, esophagus, stomach and intestines The intestines are divided into three sections:

small intestine;

colon;

rectum.

Brief description of the 16 most famous units:

Detachment single pass. Representatives: platypus, echidna and prochidna. Characterized next primitive features: the presence of a cloaca, the absence of nipples, the laying of eggs, significant fluctuations in body temperature, etc.

Order of marsupials. Representatives: kangaroo, marsupial devil, koala, wombat, etc. Characteristic: underdevelopment of the placenta, the presence of marsupial bones and a bag in which the cubs are born, the cubs are born underdeveloped.

Order of insectivores. Representatives: hedgehogs, shrews, moles, desman, etc. - the most primitive detachment of placental mammals.

Detachment woolly. Representative: woolly wing, living in Southeast Asia. Characteristic features are similarities with insectivores, bats and primates. A membrane covered with hair is developed on the sides of the body.

Order of bats. Representatives: the bats(vechernitsy, bats, horseshoe bats, vampires, etc.) and fruit bats. The forelimbs are turned into wings: the fingers are elongated and a membrane is stretched between them.

Squad of lemurs. Representatives: loris, indri, tarsier, ring-tailed lemur, etc. Occupy an intermediate position between insectivores and primates.

A detachment of primates. Representatives: monkeys, spider monkeys, chimpanzees, gorillas, humans, etc. They are characterized by a significant development of the brain, a large number of furrows and convolutions of the cortex.

Squad of rodents. Representatives: rats, mice, porcupines, squirrels, marmots, nutria and many others. The most numerous group. Animals belonging to this order are characterized by a significant development of incisors (2 each on the upper and lower jaws), there are no fangs.

Detachment hare. Representatives: hares, pikas, rabbits. On the upper jaw there are not two incisors, as in rodents, but four.

Squad of carnivores. Representatives: cats, lions, leopards, mongooses, martens, wolves, dogs, hyenas, bears, raccoons. They have poorly developed incisors, powerful fangs and molars with sharp cutting surfaces.

Detachment pinnipeds. Representatives: seals, fur seals, walruses, seals, etc. Characterized by: valky massive body, modified flippers fore and hind limbs. The teeth are usually conical in shape.

Order of cetaceans. Representatives: baleen whales (blue, bowhead whale, humpback, fin whale, etc.) - teeth are laid at the embryos, but do not develop in adult animals, a horn formation hangs into the mouth - a whalebone; toothed whales (dolphins, sperm whales, killer whales, etc.) have well-developed more or less uniform conical teeth. In all whales, the forelimbs are transformed into fins, and the hind limbs are reduced. A horizontal caudal fin is developed, as well as a dorsal fin.

Detachment artiodactyls. Representatives: pigs, hippos, bulls, giraffes, antelopes, deer, goats, sheep, etc. The greatest development of only two fingers on each foot is characteristic.

Detachment callosities. Representatives: camels, llamas. They have claw-like hooves, two-toed limbs (they used to be classified as artiodactyls).

Detachment equids. Representatives: horses, tapirs, rhinos, donkeys, etc. They are characterized by the fact that only one toe on each foot (or an unpaired number) is most developed.

Proboscis squad (elephants). Representatives: Indian and African elephants. They are characterized by a significant development of incisors (tusks), only four molars (two each on the upper and lower jaws), have a trunk, which is formed as a result of fusion of the nose and upper lip.

TO type Chordates include animals that have an internal axial skeleton - a notochord - in the adult or embryonic period of life. In the process of evolution, chordate animals have reached the highest, in comparison with other types, level of organization and flourishing. They live in all areas of the globe and occupy all habitats.

In the chordate type, 3 are distinguished subtype :

tunicates,

Cephalothordates (Skullless) - class Lancelet,

Vertebrates (Cranial) - classes Cyclostomes, Cartilaginous fishes, Bony fishes, Amphibians, Reptiles, Birds, Mammals.

The main signs of chordates:

three-layer structure

bilaterally symmetrical animals

have a secondary body cavity and a secondary mouth

the complex structure and development of the gastrula, during which the chord and the neural plate are formed. The next stage - neurula - the formation of the neural tube

have an internal axial skeleton - a chord: in non-cranial it persists throughout life, in vertebrates it is replaced by a cartilaginous or bone spine

the central nervous system has the form of a tube located on the dorsal side of the body above the chord. The cavity of the neural tube is the neurocoel. In most chordates, the anterior section of the neural tube grows and forms the brain (neurocoel - the ventricles of the brain)

the digestive tube is located under the notochord. In its anterior section there are gill slits that communicate with the external environment and persist either throughout life (non-cranial, from vertebrates - cyclostomes, fish), or only in the embryonic period of development (amphibians, terrestrial vertebrates)

the heart is located on the ventral side of the body and sends blood to the head end of the body

the outer integument has a two-layer structure and consists of the epidermis and connective tissue dermis

chordates have bilateral body symmetry, a secondary body cavity (whole), a metameric (segmental) structure of many organs.

Subtype Cephalothordates

Class Lancelets

A small group of primitive chordates, in which all the characteristics of the type are preserved throughout life (about 20 species of lancelets are known). They live exclusively in the seas, lead a bottom lifestyle (in the sand).

Classic representative – lancelet.

This is a small translucent animal with a length 5-8 cm, his body has torpedo-shaped, developed tail fin(in the form of a lancet) and paired abdominal folds(metapleural folds).

Internal axial skeleton represented by a chord covered with a dense connective tissue sheath.

Leather represented by a single layer of epidermis.

musculature clearly segmented (muscle segments are called myomers).

mouth opening surrounded by numerous tentacles.

By way of eating lancelet - filter feeder. Digestive system poorly differentiated. The pharynx is pierced by gill slits opening into the peribranchial cavity. At the bottom of the pharynx lies a glandular formation that secretes mucus. The food particles that come with the flow of water stick to the mucus and, with the help of cilia of the ciliated epithelium lining the pharynx, are sent to the intestines. The intestinal tube forms a blind protrusion - a hepatic outgrowth (similar to the real liver of vertebrates).

Circulatory system closed, developed one circle of blood circulation, no heart. The blood flow is maintained by the pulsating abdominal aorta.

excretory system It is represented by numerous paired nephridia - tubules located in segments. The excretory openings open into the peribranchial cavity.

Breath carried out with the help of gills.

lancelets - dioecious animals, their sex glands do not have their own excretory ducts.

fertilization and development larvae (with metamorphosis) occur in water.

Above the chord, the neural tube runs along the entire body. central nervous system formed by a tube with a neurocoel inside. Peripheral nerves arise from the neural tube.

sense organs poorly developed, there are light-perceiving pigment spots, a tactile cell and an olfactory fossa.