All chordates. General characteristics of chordates. Origin, structure and systematics

The chordate phylum includes a huge number of animals, including both primitive and very advanced species, from which humans originated.

Nervous system

Chordates differ from other types by having a central nervous system, located above the chord. It was originally simple, but in the process of evolution it developed to an extreme degree of complexity.

Habitat and distribution

Chordates are divided into three subphyla, and only vertebrates have managed to adapt to life both in water and on land; Tunicates, or urochordates, and tunicates live only in sea water.

general characteristics

Chordates are distinguished by bilateral symmetry: they have a special internal organ- the axial skeleton, the so-called chord, or dorsal string. The notochord is present in some animals in the embryonic or larval stage.

All chordates have a dorsal chord - a strong and at the same time elastic structure that supports their body and muscles. The mouthparts of chordates are not adapted for chewing and swallowing large prey. Content with the smallest living organisms, chordates have special organs for capturing and filtering them, the main one of which is the gill pharynx. Water drawn in by the mouth passes through the gill slits, and the nutritional microorganisms contained in it are retained. The gill pharynx, which also serves for breathing, undergoes radical changes in adult individuals.

The circulatory system consists of a pulsating organ, the heart and vessels through which blood circulates. This structure is constantly becoming more complex and becoming more and more closely connected with the respiratory system. With the exception of urochordates, which are hermaphrodites, all other chordates are heterosexual.

The phylum chordates is divided into three subphyla: urochordates, or tunicates, vertebrates and vertebrates.

Urochordates, or tunicates, include many different marine animals ranging in length from a few millimeters to 10 cm. Some are sessile, like ascidians, while others lead a free lifestyle. Externally, the body of urochordates resembles a bag. The dorsal chord is present only in the caudal part, and in some species only in the larval stage. The circulatory and nervous systems are reduced.

The structure of skullless animals has many similarities with vertebrates. The body, laterally compressed, is covered with epidermis. The dorsal chord runs through the entire body and is also present in adults. Skullless have no limbs, but they do have fins with which they swim, and various metameric organs along the entire body. In the anterior part of the abdominal cavity there is a mouth without jaws, but with numerous fibers that retain food that is filtered from the water. The closed circulatory system consists of only blood vessels: the skullless have no heart.

Fertilization in these heterosexual animals occurs in water.

Vertebrates are the most highly organized representatives of the chordate type. The dorsal chord is present only in embryos; in adults it is replaced by an axial spine, consisting of a number of cartilaginous or bony vertebrae. From this solid base extend two pairs of limbs that serve for movement. The skin is formed by the dermis and epidermis, which perform a protective function.

The nervous system consists of the brain, protected by the cranium, the spinal cord and the peripheral nervous system. The blood, pushed by the pulsating movements of the heart, circulates through a dense network of vessels, arteries and capillaries. The respiratory apparatus consists of gills in aquatic vertebrates and lungs in terrestrial ones. A rather complex digestive system is formed in different animals by different organs.

Vertebrates are heterosexual and can be ovoviviparous (females lay eggs), ovoviviparous (eggs develop in the female’s body) and viviparous (the embryo develops in the uterus, receiving nutrition directly from it).

Kinds

Ascidia belongs to the subphylum Urochordate. A very primitive sac-shaped animal, “dressed” in a dense “dead end” - living tissue with two openings: an oral siphon adapted for absorbing water, breathing and retaining nutrient particles, and a cloacal siphon for releasing waste.

One of the most primitive vertebrates. It has a rocker-shaped body without jaws or limbs. The mouth is a funnel, studded with numerous molars, which are located concentric circles, in the center of these circles is the tongue. The lamprey tenaciously, like a suction cup, attaches its mouth to its victim and sucks blood from it.

Lancelet from the subtype of skullless - a small, almost transparent marine animal that lives on the bottom of the sea and represents a transitional species between invertebrates and vertebrates. The dorsal nerve chord and tail muscles of the lancelet are similar to the chord and muscles of fish, but, on the other hand, it has neither sensory organs, nor jaws, nor a skeleton.

Grade: 7th grade

Date of: __________

Lesson topic: “Brief description of the phylum of chordates. Lancelet is a representative of the skullless. Habitat and structural features of the lancelet. Role in nature and practical significance"

Lesson type: combined

The purpose of the lesson: study of the characteristics of chordates using the example of the lancelet, their origin.

Tasks:

Educational : introduce students to characteristic features subtype cranial and with adaptations to life in an aquatic environment.

Developmental : formation of interest in the animal world, activation of mental activity, development of creative thinking.

Educational : using the lancelet as an example, form careful attitude to rare animals and the basics of ecological behavior.

Equipment: textbook "Biology" 7th grade. "AtaMura» 2012, workbook"Biology" 7th grade. "AtaMura"2012, presentation for the lesson, table "Type Chordata".

Preparation for work (checking students' readiness for the lesson)

Checking student availability

Greetings from the teachers.

Dep. report

Check of knowledge

Verification work on the topic "Class Insects".

They write other work

Learning new material

Today we will get acquainted with a new type of living beings - Type Chordata.

(Write the number and topic in a notebook).

In water and on land, in soil and air, and even in the organs of plants, animals and humans, a wide variety of animals live everywhere on earth. Currently, there are about 2 million species of animals.
Animals on our planet vary in size and body shape. They differ in the structure of body parts, integument, limbs, and sensory organs.
Most animals can move with the help of legs, wings, flippers, and fins. Many do not have organs of movement and lead an attached or sedentary lifestyle. Animals differ not only in appearance, but also in internal structure and behavior. Today we will visit the world of Animals - the world of Chordates.
Chordates are a large group of highly organized animals with bilateral body symmetry. Chordates occupy all habitats. Currently there are more than 40 thousand species.

What animals are called chordates? Why? (state hypotheses, look at the diagram on the slide).

The notochord is an elastic cord in chordates that lies above the intestines.

Exercise : read the text on page 181 and answer “What are the characteristics characteristic of chordates?”

Traits of chordates :

Presence of chord (in lower representatives the notochord remains throughout life, in higher representatives it is replaced by the spine).

Nervous system -neural tube (above the chord)

Availabilitygill slits (preserved in lower chordates, aquatic animals and amphibians; in chordates living on land, they are formedlungs )

Chordates comprise 3 subphyla:

Vertebrates (cyclostomes, fish, amphibians, reptiles, birds, mammals)

Cephalochordates (lancelet)

Tunicates (sea squirts, appendiculars, salps)

We will look in more detail at the Skullless subtype, of which the Lancelet is a representative. In 1774, Pallas discovered the lancelet and classified it as a type of Mollusca.

In 1834, Kovalevsky proved that the lancelet is a transitional form between invertebrates and vertebrates.Appearance. The animal got its name from its external shape, similar to a surgical instrument - a lancet. A translucent animal with a length from 1 to 8 cm. Lancelet is classified as a lower chordate. Habitat and lifestyle. Temperate and warm seas, live at depths of 10 to 30 m.Movement. Most spends time half-buried in the ground, with the front end of the body exposed, surrounded by a halo of tentacles. The disturbed one swims a short distance and buries itself again.Nutrition . Typical filter. Food that enters the pharynx with water is retained in a special groove (endostyle), where food boluss form. With the help of ciliated epithelium they are directed into the intestinal tube. The food of the lancelet is: diatoms, protozoa, cladocerans, larvae of lower animals.

Let's consider internal structure lancelet (page 182)

Exercise : read the text on page 182 and fill out the table (RT page 70 No. 302) (Checking the table)

Structure, organ system

Peculiarities

Body Shape

Laterally compressed, translucent, length 5-8 cm.

Skeleton

Chord

Digestive

Mouth, peripharyngeal tentacles, pharynx, hepatic process, intestines (middle and posterior), anus

Respiratory

Gill slits

Blood

Closed. There is no heart.

excretory

Anal hole

Nervous

Neural tube

Sexual

Dioecious. External fertilization

What is the significance of the lancelet in nature and in human life? ?

The lancelet feeds on microscopic organisms, drawing them in through its mouth opening with a stream of water. The lancelet's food consists mainly of diatoms, as well as desmidiaceae, small rhizomes, ciliates, radiolaria, eggs and larvae of tunicates, echinoderms, crustaceans, etc. Thus, these are the main biofilters of the bottom layer of water.

The Asian lancelet is the object of a special fishery practiced in autumn and winter (August - January), known for 300 years. It is fished in the southwestern part of the East China Sea. Lancelet is caught from boats for 2-4 hours during low tide, scooping up the top layer of sand with a special shovel on a long bamboo stick. Carefully lifting the shovel, shake off the scooped up sand onto a washing tray and then onto a sieve, separating the lancelets from the sand and shells. One boat usually produces about 5 kg of lancelet per day. Lancelet meat contains 70% protein and about 2% fat.

Locals They make soup from the lancelet and fry it. Part of the catch is dried over low heat and exported to the island of Java and Singapore. The annual lancelet catch is approximately 35 tons, which corresponds to 280 million individuals. Lancelet is sometimes used as food on the island of Sicily and in Naples.

Now let's look at the subphylum Vertebrates or Cranials.

Signs of the subtype Cranial, or Vertebrate.

Skeleton of the head, or skull

Spine, made up of vertebrae

Brain and spinal cord

Sense organs – hearing and vision become more difficult

Developed muscles

Paired limbs

Closed circulatory system, heart

Breathing - gills and lungs.

Active lifestyle.

Exercise: RT pp. 70-71 No. 304, 305, 307, 308, 309.

Write down the date and topic in your notebook

They express an opinion.

Write down the definition of “chord”.

Do the assignment according to the textbook. Write it down in a notebook.

Representatives of the subtypes are named and written down.

Look at the drawing. Fill out the table.

Updating the acquired knowledge

Task “Identify the organs of the lancelet”:

Check by key on the slide:

Neural tube

Chord

Intestines

Gill slits

Periopharyngeal tentacles (oral opening)

Frontal conversation

Why is the phylum called Chordata?

List their main features.

What subphyla are chordates divided into?

What do you know about lancelets?

What groups are vertebrates divided into depending on the presence of jaws?

Complete the assignment, peer review, and grade

Reflection.

Bottom line

Reflection “Traffic Light”:

Red – I didn’t understand anything in the lesson.

Yellow – not everything is clear.

Green - everything is clear to me.

Summing up the lesson

Grading

Summarize the lesson.

D/ h

Learn notes in a notebook

Page 182 drawing “Structure of the lancelet”

Record d.z.

The phylum includes about 43 thousand species of animals that inhabit 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 of 2-3 cm to giants (some species of whales reach a length of 30 m and a mass of 150 tons).

Despite the huge diversity of representatives of the phylum Chordata, they are characterized by common features organizations:

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 lower groups 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 is transformed into the brain; in the trunk and caudal sections it is represented by the spinal cord,

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

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

Rice. Diagram of the structure of a chordate animal

5. Apart from these distinctive features, characteristic only of chordates, they have the following characteristics: all of them are bilaterally symmetrical, deuterostome, deuterostome animals.

6. The phylum Chordata is divided into three subphyla and 12 classes. Let's consider the most important of them.

Subphyla and classes of chordates

The phylum of chordates includes three subtypes—Craniolates, Larval chordates, and Vertebrates. Chordata have an internal skeleton called notochord in the early stages of development. Chordates occupy the main living environments: aquatic, ground-air and soil. These are bilaterally symmetrical three-layered animals. Chordates include fish, amphibians, reptiles, birds, and mammals.

Subtype skullless

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.

Subphylum vertebrates

Class cartilaginous fish

The class of cartilaginous fish includes about 660 species. This group includes the well-known sharks (fried shark, tiger shark, katran) and rays (stingfish, sawfish, manta ray), combined into two separate superorders, as well as whole-headed ones (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.

The class of bony fishes is the largest group of vertebrates. It has about 20,000 species belonging to 4 subclasses: ray-finned, multi-finned, lobe-finned, lungfish.

We list the main representatives of the class:

order sturgeon - beluga, sturgeon, sterlet;

order Salmonidae - salmon, salmon, trout;

order carp-like - bream, carp, crucian carp, silver carp;

order codlike - cod, hake, pollock;

order perciformes - perch, horse mackerel, mackerel, pike perch.

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

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

Method of transportation - swimming;

Streamlined body shape;

Fixed articulation of the head with the body;

The scales are imbricated;

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

Breathing using gills;

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 adult individuals can live both on land and in water. However, their reproduction and egg development almost always occurs in an aquatic environment. This class includes about 3,000 species, divided into three orders:

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

The order caudate 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, spadefoots, and fire-bellied toads.

Almost all amphibians are small in size. The body of adult individuals is divided into a head, a trunk, a tail (in the order caudates) and two pairs of limbs (in caecilians the limbs and their girdles are reduced). In connection with reaching land, the body of most is flattened in the dorso-abdominal direction, and the head is movably articulated with the body. Amphibians have bare skin, so water and gases can diffuse through it unhindered.

Class reptiles or reptiles

There are about 6,600 species of reptiles in the world's fauna. Living reptiles are grouped into the following groups:

Turtle Squad (representatives: snapping turtle, green turtle);

Order Beaked (a very ancient group with a single surviving species - the tuateria, which is found in New Zealand. Among modern reptiles, the tuateria is closest to the order Squamate;

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

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

Representatives of the class reptiles are true terrestrial animals. The development of adaptations to 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 beak-headed fish) there are forms that have secondarily transferred to life in water.

Bird class

This class unites about 8,600 species of living birds. They are divided into two superorders. Superorder Penguins (or Swimming). Representatives of this group (king penguin, little penguin, Galapagos penguin, etc.) are large animals, they cannot fly, the main method of movement is swimming. The forelimbs are modified into flippers. Penguins are common in cold regions of the southern hemisphere - in Antarctica and the islands of the Subantarctic. The superorder New palate, or Typical birds, is represented by big amount orders: ostriches, anseriformes, gallinaceans, cranes, bustards, waders, gulls, owls, woodpeckers, parrots, passerines, etc. Almost all the characteristics of birds are associated with their exploration 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 feathers. The axial part of the feather is the shaft and the blade. The tip of the feather is immersed in the skin, and fans extend from the shaft. In a contour feather, 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 shaft of the down feather is thin, there are no hooks. A down feather, the beards of the first order of which extend in a tuft from the feather, is called down. Contour feathers give the body a characteristic shape, and down feathers serve as heat-insulating material. Periodically, birds change their plumage - molting.

Class mammals (or animals)

The class mammals is the last of the classes of vertebrate animals we are considering, which represent 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 the mountains and ocean spaces.

It would seem that they belong to this class completely similar friend animals on each other: the blue whale and the common hedgehog, the African elephant and the squirrel, the bat and the kangaroo, etc. You and I also systematically belong to this group. What, however, 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 cubs with milk and caring for the offspring;

viviparity (with the exception of monotremes);

constant body temperature - homeothermicity;

intensive occurrence of basic life processes;

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

the lungs have 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;

Most species have seven cervical vertebrae (the exceptions are dugongs, manatees and sloths);

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

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

I subclass - cloacal (oviparous or protobeasts) with one order - monotreme;

Subclass II - marsupials with one order marsupials;

III subclass - placental (or higher animals) with nineteen orders: insectivores, chiroptera, woolly winged animals, prosimians (lemurs), monkeys (primates), odontates, lizards (pangolins), aardvarks, rodents, lagomorphs, carnivores, pinnipeds, cetaceans, artiodactyls , calloused, odd-toed ungulates, hyraxes, proboscis (elephants), lilac (sea cows).

Among the representatives of this class, body size and weight vary within very wide limits. The smallest animal in the world fauna, the little 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 (the skin itself), as well as a layer of connective tissue, which may contain (sometimes significant) fat accumulations. Animals of this class are characterized by a large number of horny formations, which include:

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

scales (in pangolins);

horny plates (armadillos);

horns of rhinoceroses, horn sheaths of bovids (cows, goats);

nails (human and other primates);

claws (predators, anteaters);

hooves (horses, cows, tapirs, hippos).

Often hairline It can be highly developed and form thick fur. There are two types of hair:

Long and relatively sparsely located, called ostia;

Short and dense, called undercoat.

The skin is rich in glands, among which there are sebaceous and sweat glands. The sebaceous glands have a grape-shaped body, from which channels extend that open in the hair follicle. These glands secrete an oily secretion. Sweat glands look like coiled tubes that open on the surface of the body. The milk and odorant glands are modified sweat glands. The mammary glands, which secrete milk necessary for feeding offspring, have a grape-shaped structure and open at the nipples. In monotremes (platypus, echidna), these glands have a tubular structure and open not on the nipples, since there are none, but in the hair follicles. Baby platypuses and echidnas simply lick droplets of milk from their mother's fur. The skeleton has a number of features. The surface of the vertebrae is smooth, and not saddle-shaped, like in birds, and not convex-concave, like in reptiles. The spinal column is divided into five sections:

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

Thoracic (counts from 9 to 24, usually 12, vertebrae);

Lumbar (2-9 vertebrae);

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

Caudal (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 scapula, forming the coracoid process. The free forelimb consists of: humerus, ulna and radius bones, carpus, metacarpal bones and phalanges. The hind limb girdle (pelvic) is represented by the pelvic bones (ischium, pubis and ilium). The free hind limb consists of the femur, tibia, fibula, tarsus, metatarsal bones and phalanges.

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 characteristics of the 16 most famous units:

Order monotremes. Representatives: platypus, echidna and echidna. They are characterized by a number of primitive features: the presence of a cloaca, absence of nipples, laying eggs, significant fluctuations in body temperature, etc.

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

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

Order Woolwings. Representative: woolly wing, living in Southeast Asia. Characteristic features of similarity with insectivores, bats and primates. A hairy membrane is developed on the sides of the body.

Order Chiroptera. Representatives: the bats(bats, 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.

Lemur squad. Representatives: loris, indri, tarsier, ring-tailed lemur, etc. They occupy an intermediate position between insectivores and primates.

Primate order. Representatives: monkeys, spider monkeys, chimpanzees, gorillas, humans, etc. They are characterized by significant development of the brain, a large number of grooves and convolutions of the cortex.

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

Order Lagomorpha. Representatives: hares, pikas, rabbits. On the upper jaw there are not two incisors, like in rodents, but four.

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

Order Pinnipeds. Representatives: seals, fur seals, walruses, ringed seals, etc. Characterized by: a ridged massive body, modified flipper-like fore and hind limbs. The teeth are usually conical in shape.

Order Cetaceans. Representatives: baleen whales (blue whale, bowhead whale, humpback whale, fin whale, etc.) - teeth are formed in embryos, but do not develop in adult animals, a horny formation hangs into the mouth - 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.

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

Calloused squad. Representatives: camels, llamas. They have claw-like hooves and two-toed limbs (previously they were classified as artiodactyls).

Order Perissodactyls. Representatives: horses, tapirs, rhinoceroses, donkeys, etc. They are characterized by the fact that only one toe on each foot (or an unpaired number of them) is most developed.

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

Chordates are a phylum of animals characterized by a variety of representatives that have achieved evolutionary progress. Nowadays, there are about 60 thousand species of chordates. These include fish, frogs, lizards, birds, animals, etc. Representatives of chordates live in water, on land, in the air and soil. In the course of evolution, they have adapted to the most various conditions environment.

Despite their diversity, all chordates have general plan structure, which resembles the general plan of structure of most invertebrates, inverted in the dorsoventral direction. In chordates, the neural tube is located above the intestines (and notochord or spine), blood flows on the ventral side of the body from the tail to the head, on the dorsal side - from the head to the tail. And most invertebrates have an abdominal nerve cord; blood flows in the opposite direction compared to chordates (on the back - from tail to head, on the belly - from head to tail).

Home distinctive feature chordates is the presence of an internal axial skeleton. In the most primitive representatives (lancelet, some groups of fish), the role of the axial skeleton is played by the notochord, which looks like a dense, elastic, but quite elastic longitudinal cord (rod). It consists of cartilage-like tissue. In most of the more highly organized representatives of chordates, during embryonic development, a vertebral column (spine) develops in place of the notochord. It can be cartilaginous or bone. The notochord is formed from a longitudinal outgrowth of the intestinal tube on the dorsal side, i.e., it is of endodermal origin.

It was in the Chordata type that the nervous system reached its highest development. A tubular nervous system is characteristic. The neural tube begins above the notochord and is of ectodermal origin. In most, the anterior portion of the neural tube expands to form the brain. In this case, the neurocoel (the cavity of the neural tube) is transformed into the ventricles of the brain.

The digestive tube is located under the notochord, and below the digestive tube is the heart (or similar vessel).

Gills are formed not on the surface of the body, but inside it - in the pharynx. Thus, the pharynx becomes pierced by gill openings. They are preserved in all proto-aquatic chordates; in the rest, gills are present only in embryos in the initial stages of their development.

The circulatory system of chordates is closed.

Chordates belong to the group of deuterostome animals, since during embryonic development their mouth is formed not from the side of the invagination of the blastula, as in most invertebrates, which are therefore called protostomes, but from the opposite side. At the site of invagination of the blastula in chordates, the anus develops.

All chordates belong to secondary cavities.

The phylum Chordata includes three subphyla. These are cephalochordates, or skullless (lancelets), larvalochordates (tunicates), and vertebrates, or craniates (all others). In tunicates, the notochord is present only at the larval stage. The most numerous subtype in terms of species composition and prevalence is Vertebrates.

Chordates are the highest type of animal kingdom, uniting more than 43,000 species of very diverse sizes, appearance and habitat. Most of them have an internal cartilaginous or bony skeleton and are called vertebrates. The structural plan of vertebrates sharply distinguishes them from animals of other types, and therefore the question of the origin of vertebrates for a long time remained unresolved. The credit for clarifying this issue belongs to the Russian embryologist A.O. Kovalevsky (1840-1901). Having studied the development of the embryos of some primitive marine animals - the lancelet and ascidians, he showed that they are transitional forms that combine the features of invertebrates and vertebrates.

Thanks to this, A. O. Kovalevsky managed to overcome the gap that separated vertebrates from the lower groups of the animal world and shed light on their origin. Currently, vertebrates, together with transitional forms, are united into a single type of chordates.

Type characteristics

Despite the diversity of species, all chordates have a common structural plan and differ from representatives of other types in the following four main characters.

1. They have an internal axial skeleton, represented by a dorsal string, or chord (chorda dorsalis). The chord is an elastic flexible rod. It develops from the endoderm, consists of highly vacuolated cells and is surrounded by a connective tissue membrane. In lower chordates, it is preserved throughout life (lancelets, sturgeons, lungfishes and lobe-finned fish), in higher chordates (i.e., most vertebrates), the chord is present only in larvae or embryos, and then is replaced by a cartilaginous or bone formation - the spine . The spinal column consists of individual vertebrae, which during the process of ontogenesis are formed in the connective tissue membrane of the notochord.
2. The central nervous system is located on the dorsal side above the notochord. It looks like a tube stretching along the body and has an internal cavity - a neurocele. The central nervous system develops from the ectoderm and differentiates in vertebrates into the brain and spinal cord. In all invertebrates, the nervous system is located on the ventral side of the body and is a chain of nerve ganglia connected by nerve cords.
3. The digestive system is located under the notochord, it begins with the mouth and ends with the anus (anus). The anterior (pharyngeal) section of the digestive tube has a number of through holes - the gill apparatus. It is represented by gill slits that pierce the wall of the pharynx, and a skeleton that supports the gill slits (visceral arches). The gill apparatus, as well as the notochord, is not preserved in all adult animals. Gill slits are characteristic of fish throughout life and are supplemented by special organs of aquatic respiration - gills; in others they are present only in the larval state (tadpoles of amphibians); in terrestrial vertebrates, gill slits are formed in the embryo, but soon become overgrown; the air respiratory organs - the lungs - develop as paired protrusions on the ventral side of the back of the pharynx.

   The notochord, neural tube and intestine stretch along the entire body and form a complex of axial organs.

4. The circulatory system is closed. Central authority blood circulation - the heart or a pulsating blood vessel replacing it - is located on the ventral side of the body and is formed in the embryo under the notochord and the digestive tube.

In addition to these basic characteristics of chordates, they are very characterized by the mutual arrangement of the nervous, supporting and digestive systems. In other types of the animal kingdom there is no such strict pattern (remember the location of the corresponding systems of round and annelids, arthropods, mollusks).

At the same time, chordates have characteristics in common with achordates. All chordates have bilateral symmetry, have a metameric organ anlage, a secondary body cavity and a secondary mouth.

The oral opening is formed by breaking through the gastrula wall. At the site of the opening of the gastrula (gastropore), an anal opening is formed. This character unites chordates, echinoderms, and several few related types into the group of deuterostomes. The remaining types discussed earlier (with the exception of unicellular ones) are combined into the group of protostomes.

The main organ systems of chordates, including the skeleton, muscles, nervous system, excretory organs, etc., are formed metamerically in embryos. Metamerism is especially pronounced in the embryonic period.

The chordate phylum is divided into 3 subphyla.

• Subphylum Tunicata (tunicates).

The first two subtypes include a small number of primitive marine animals that lead a sedentary or motionless lifestyle and lack a vertebral column. There are no tunicates in the program for entering universities. Cranials have a nervous system structure typical of chordates in the form of a neural tube, but its anterior part is not expanded and not protected by any cartilaginous or bone formation, i.e. there is no brain or skull. The skullless subtype includes one class of simply constructed animals - lancelets. These are not numerous (only about two dozen species) marine animals.

When characterizing the lancelet, pay attention to the features that make it similar to lower invertebrate animals: the absence of a brain, real sense organs and heart, the absence of paired limbs, the primitive structure of the excretory organs. On the other hand, it is necessary to emphasize the signs of a progressive organization: the presence of a true notochord and a tubular nervous system typical of chordates, as well as a closed circulatory system. Listed signs suggest that the lancelet and vertebrates had a common ancestor, which was probably similar to skullless animals.

The fourth subtype is the most numerous. It unites highly organized animals with a cartilaginous or bony skeleton. The anterior part of the neural tube is expanded, forming the brain, which is protected by a cartilaginous or bony skull. The part of the neural tube located in the body and called the spinal cord is enclosed together with the notochord in a cartilaginous or bony spine, consisting of individual vertebrae. There is a circulatory organ - the heart, which lies on the ventral side of the body, as well as complex kidneys. In addition, vertebrates are characterized by well-developed paired limbs (except for cyclostomes) and perfect sensory organs (vision, hearing, smell, etc.). All this provides vertebrates with high mobility, the ability to navigate in space, and easily find prey.

The vertebrate subphylum is divided into six classes: cyclostomes, fish, amphibians (amphibians), reptiles (reptiles), birds and mammals. The applicant is required to know the last five classes. Their a brief description of is given in table. 16.