Symmetry of chordates. General characteristics of chordates. Origin, structure and systematics. What have we learned

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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Chordates include about 40,000 various kinds individuals that differ from others in structure, lifestyle, habitat.

The Paleozoic era contributed to the appearance of this type of animal about 500 million years ago. Scientists suggest that their ancestors were annelids.

Chordates settled all over the planet and became habitual inhabitants of the sea, land, air and even soil.

What is a chord and who are chordates

The internal structure of chordates is different from others. They are characterized by the presence of an axial skeleton - the spinal column, which is otherwise called the chord.

It is this feature of the structure of the spine that gave the name to chordates.

Structural features

The following features are characteristic of chordates:

The location of the neural tube above the axial skeleton and the formation of the spinal cord from it.
The presence of a rod - a chord.
Absence of intestines in the caudal region.
Location of the heart under the digestive tract.

Type Chordates (chordata) - examples of animals

Representatives of chordates:

Origin and evolution of chordates

Biology as a science considers the origin of chordates to be one of the most important stages in the development of the historical animal world.

The emergence of this type meant the emergence of new animals with a unique structure, which allowed them to further evolve to creatures with maximum complexity in structure and behavior.

Some scientists believe that chordates began to exist even before the appearance of annelids, which fed by filtration. Other scientists attribute them to the ancestors of chordates.

One way or another, but the evolution of annelids, or, as they are also called, bottom worm-like animals, gave birth to new types: echinoderms, pogonophores, hemichordates and chordates.

Subsequently, chordates evolved in three directions, depending on the lifestyle:

The habitat of individuals of the first direction was hard ground. This contributed to the active development of the filtering apparatus, which provides nutrition during a sedentary lifestyle, and the formation of a thick protective shell on the entire surface of the body. These individuals have the ability to reproduce asexually. This is how shellers appeared.
The habitat of individuals of the second direction is the bottom. They moved a little more, digging into the ground. This way of life simplified their original organization. The development of the myochordial complex required increased mobility, and the expansion of the pharynx added new gill slits. This branch has survived to this day in the form of non-cranial.
The habitat of individuals of the third direction, which began to lead a floating lifestyle, is fresh water. There has been a transition to active nutrition, increased mobility. The nervous system and sense organs became more perfect, which led to the complication of behavior and the emergence of more complex forms. Thus, a group of vertebrates appeared.

In rivers and other fresh waters, jawless ones also formed, from which jawed ones later separated. They expanded their habitat into salt water and became the progenitors of modern fish groups.

Later, amphibians separated from the fish. Then they came to land, and thus a new species appeared - reptiles.

General characteristics of the chordate type

The cover consists of two layers of leather. The upper layer is represented by the epidermis and its derivatives: scales, feathers, wool, hair. In this layer of the skin are odorous glands that produce mucus and sweat. The lower layer is the dermis, which is made up of fibrous connective tissue.

The musculoskeletal system is presented in the form of a skeleton, consisting of a chord and a membrane connecting the tissues. The skeleton of the head is divided into brain and facial parts.

Fish develop jaws, while vertebrates develop two pairs of limbs. Bones are connected by joints.

The respiratory system in lower chordates is represented by gills, while in vertebrates it is represented by lungs. In addition, the skin of chordates is partially involved in gas exchange.

The digestive system in cephalochords is a straight tube and almost undeveloped digestive glands. In vertebrates, this is the alimentary canal, which has sections.

First, food enters the mouth, then passes into the pharynx, begins to be processed in the esophagus, passes into the stomach, and finally enters the intestines. In addition to these organs, vertebrates have a liver and pancreas.

The circulatory system is closed. In vertebrates, due to the increase in the intensity of metabolism, the heart appeared and became more complicated. Cephalothordates have no heart.

In birds, the heart differs from the heart of reptiles only in the presence of a complete septum and the absence of the left aortic arch; mammals have a four-chambered heart that pumps two types of blood: arterial and venous.

The central nervous system (CNS) of chordates has the form of a neural tube with an internal canal, which in vertebrates forms the brain. The peripheral nervous system consists of cranial and spinal nerves that branch off from the central nervous system.

The excretory system in all chordates, except for lancelets, is represented by paired kidneys, ureters and bladder.

Reproductive system: reproduction occurs with the help of testes in males and ovaries in females. Tunicates are hermaphrodites, they reproduce both sexually and asexually. The rest of the chordates have a sexual division.

Classification of chordates and their subtypes

Chordates are divided into lower (lamprey, lancelet, hagfish) and higher (reptiles, amphibians, fish, birds, mammals).

The following subtypes are distinguished:

non-cranial;
tunicates;
jawless;
primary water: classes of fish;
tetrapods: classes of amphibians or amphibians, reptiles or reptiles, birds, mammals.

What signs of chordates does a person have

In humans, like chordates, early stages development, the formation of the axial skeleton, i.e., the chord, occurs. The musculoskeletal system in humans is represented, as in vertebrates, by the supporting internal skeleton.

Man also has the following features of chordates:

the central nervous system, which has a tubular structure;
a closed circulatory system with the main organ of blood circulation - the heart;
breathing apparatus capable of communicating with the external environment through the pharynx, nasal cavity and mouth.

monkey eel

Some interesting information:

The value of chordates is very great, they are classified as the most diverse and numerous types. At the moment, there are about 50 thousand species of chordates. The presence of a common feature in all individuals - the chord (supporting organ) gave the name to this type of animal.

Anatomical signs of chordates are similar to echinoderms. The lower representatives of chordates are lancelets, which retain their main feature throughout their lives.

common features of the chordate type

The chordate phylum is subdivided into three subtypes: non-cranial, tunicates, and vertebrates. Despite the great variety of species, the body of all chordates has a common structural plan and consists of a head, trunk, tail and limbs. Main Feature representatives of the type is the presence (at least at one of the stages of individual development) of a chord of a flexible, elastic cord that acts as an axial skeleton. The notochord is located above the intestine and is formed from the endoderm by splitting off a cell cord from the dorsal side of the intestinal tube.
Despite the exceptional diversity of chordates, they all have a number of common structural and developmental features. The main ones are:
1. All chordates have an axial skeleton, which initially appears in the form of a dorsal string, or chord. The notochord is an elastic, non-segmented strand that develops embryonically by lacing it from the dorsal wall of the germinal gut: the notochord is of endodermal origin. The subsequent fate of the chord is different. It persists for life only in lower chordates (with the exception of ascidians and salps). In most representatives, the notochord is reduced to one degree or another in connection with the development of the spinal column. In higher chordates, it is an embryonic organ and in adult animals it is to some extent replaced by vertebrae, in connection with this, the axial skeleton from a continuous non-segmented cord becomes segmented. The spine, like all other skeletal formations (except the notochord), is of mesodermal origin and is formed from a connective tissue sheath surrounding the notochord and neural tube.
2. Above the axial skeleton is the central nervous system of chordates, represented by a hollow tube. The cavity of the neural tube is called the neurocoel. The tubular structure of the central nervous system is characteristic of almost all chordates. The only exceptions are adult tunicates. In almost all chordates, the anterior neural tube grows and forms the brain. The internal cavity is preserved in this case in the form of the ventricles of the brain. Embryonally, the neural tube develops from the dorsal part of the ectodermal bud.
th. In lower forms, gills are located on their walls. Gill slits are preserved for life only in lower aquatic chordates. For the rest, they appear only as embryonic formations, functioning at some stages of development or not functioning at all.
Along with these three main features of chordates, the following should be mentioned. specific traits their organizations, which, however, besides the chordates, are also found in representatives of some other groups. Chordates, like echinoderms, have a secondary mouth. It is formed embryonically by breaking through the gastrula wall at the end opposite the gastropore. In place of the overgrown gastropore, an anus is formed. The body cavity in chordates is secondary (as a whole). This feature brings chordates closer to echinoderms and annelids.
The metameric arrangement of many organs is especially pronounced in embryos and lower chordates. In their higher representatives, due to the general complication of the structure, metamerism is weakly expressed.

Chordates are characterized by bilateral (bilateral) symmetry of the body. As is known, in addition to chordates, many groups of invertebrates have this feature.
Evolutionarily, chordates are characterized by morphophysiological continuity in all organ systems, which can be traced in the change in homologous organs.
All chordates have an axial skeleton or notochord.
3. The anterior (pharyngeal) section of the digestive tube communicates with the external environment by two rows of holes, called visceral slits
. All chordates have an axial skeleton or notochord.
2. Above the axial skeleton is the central nervous system of chordates, represented by a hollow tube. In almost all chordates, the anterior neural tube grows and forms the brain.
3. The anterior (pharyngeal) section of the digestive tube communicates with the external environment by two rows of holes, called visceral slits
the presence of a chord instead of a spine. lack of bones
Despite the huge diversity, all representatives of the Chordata type are characterized by common features of organization that are not found in representatives of other types. Consider the main features of the type using an interactive scheme:
The body is bilaterally symmetrical.
Intestine through.
Above the intestines is a notochord.
Above the chord, on the dorsal side of the body, is the nervous system in the form of a neural tube.
The walls of the pharynx have gill slits.
The circulatory system is closed. Heart on the ventral side of the body, under the alimentary canal.
They live in all environments.

Characteristics of the type and system. The chordate phylum is often referred to as the highest phylum of animals. This is not entirely accurate, since chordates crown only a branch of deuterostomes ( Deuterostomy), while the top of the protostomes branch ( Protostomia) occupy types: arthropods ( Arthropoda) and shellfish ( Mollusca). The development of both branches followed different paths and led to the development of fundamentally different, but biologically highly active and complex types organization of living matter.

Type Existence Chordata was substantiated by the famous Russian zoologist A. O. Kovalevsky, who, studying the development (ontogenesis) of tunicates ( Tunicata) and non-cranial ( Acrania), established the fundamental similarity of their organization with vertebrates. The name of the chordate type was proposed by Ball in 1878. Now the chordate type is accepted in the following volume (extinct groups are marked with a cross (†).
The non-cranial and tunicate subtypes are usually called the lower chordates, contrasting them with the higher chordates - the vertebrate subtype.

Approximately 43,000 species belong to the chordate type. modern species distributed throughout the globe: they inhabit the seas and oceans, rivers and lakes, continents and islands. The external appearance of chordates is very diverse (fixed saccular ascidians, somewhat similar to worms, non-cranial, vertebrates of various appearance). The sizes are also different: from appendicularia a few millimeters long, small fish and frogs 2-3 cm long to giants - some whales reaching 30 m in length and weighing up to 150 tons.

Despite the huge diversity, all representatives of the chordate type are characterized by common organizational features that are not found in representatives of other types:

1. The presence throughout life or at one of the phases of development of the dorsal string - the chord (chorda dorsalis), which plays the role of the internal axial skeleton. It has an etiodermal origin and is an elastic rod formed by highly vacuolated cells; the notochord is surrounded by a connective tissue sheath. In most vertebrates, in the course of individual development (ontogenesis), the notochord is replaced (displaced) by the vertebral column, which consists of individual vertebrae; the latter are formed in the connective tissue sheath of the chord.

2. The central nervous system has the shape of a tube, the inner cavity of which is called the neurocoel. The neural tube is ectodermal in origin and lies above the notochord. In vertebrates, it is clearly differentiated into two sections: the brain and spinal cord.

3. The anterior part of the digestive tube - the pharynx - is lined with gill openings that open outward and performs two functions: a section of the digestive tract and a respiratory organ. In aquatic vertebrates, specialized respiratory organs, the gills, develop on the partitions between the gill slits. In terrestrial vertebrates, gill slits form in embryos but soon close over; specific organs of air respiration - the lungs - develop as paired protrusions on the ventral side of the back of the pharynx. The digestive tract lies under the notochord.

4. The pulsating part of the circulatory system - the heart - is located on the ventral side of the body, under the chord and digestive tube.

In addition to these typical features, chordates have some features that are also found in other types.

1. By breaking through the wall of the gastrula, a secondary mouth is formed; in the region of the primary mouth (gastropore), an anus is formed. This feature combines chordates with hemi-chordates, echinoderms, chaetognaths and pogonophores into a group of deuterostomes - Deuterostomia, opposed to the group of protostomes - Protostomia, in which a mouth opening is formed at the site of the gastropore, and the anus is formed by breaking through the wall of the gastrula (all other types of animals, except sponges, coelenterates and protozoa, belong to the protostomes).

2. In the process of embryonic development, a secondary body cavity is formed - the whole, but all deuterostomes, annelids, mollusks, arthropods, bryozoans and brachiopods also possess it.

3. The metameric or segmental arrangement of the main organ systems is especially pronounced in arthropods and many worms. Metamerism is also clearly expressed in chordates, but in adult terrestrial vertebrates it manifests itself only in the structure of the spinal column and some muscles, in the origin of the spinal nerves, and partly in the muscles of the abdominal wall.

4. Chordates, like most other multicellular animals, are characterized by bilateral (bilateral) symmetry: only one plane of symmetry can be drawn through the body, dividing it into two halves, which are mirror images of each other.

Thus, the chordate phylum unites deuterostomes, bilaterally symmetrical coelomic animals with metamerism, expressed mainly in the early stages of embryonic development. They have an internal skeleton in the form of a chord with a neural tube lying above it; and under the chord is the digestive tube. The anterior end of the latter - the pharynx - is pierced by gill slits that open outwards. The heart lies on the ventral side of the body under the digestive tube. In higher chordates, the notochord is replaced by the vertebral column; in terrestrial classes, the gill slits overgrow and new respiratory organs develop - the lungs.

Origin of chordates. Fossil remains of the ancestors of chordates have not been preserved. Therefore, it is necessary to judge the early stages of their evolution largely by indirect data: by comparing the structure of adult forms and a comparative study of embryonic development.

Ancestors of chordates were searched among various groups of animals, including among annelids. For example, it was proposed to consider some sedentary polychaetes (polychaete worms) like modern ones as the ancestors of chordates. Sabellidae and Serpulidae. These hypothetical ancestors of the chordates were supposed to have passed to active image life, but began to move on the original dorsal (dorsal) side of the body. The anal groove characteristic of these worms, which stretches forward through the glandular field along the abdominal surface, could close, forming a neural tube connected by a neuro-intestinal canal with the intestinal tube, and the glandular cells, having become part of the neural tube, ensured the neurosecretory function of the nervous system. In this case, the connective tissue cord, which in some polychaetes lies in the thickness of the ventral muscles, could become the precursor of the notochord. The probability of such formation of the internal skeleton, it would seem, is confirmed by the formation of a cartilaginous gill skeleton in some modern polychaetes. The loss of polymeric segmentation (eg, enterophans) is a secondary phenomenon from this point of view (Engelbrecht, 1969).

These witty ideas have not been confirmed. Most zoologists believe that the predecessors of chordates were apparently coelomic worm-like animals that switched to a sedentary or sedentary lifestyle, which led to a decrease in the number of segments of their body (probably to three) and the formation of a secondary mouth. They fed passively by filtering the water. These oligomeric inhabitants of the seabed, evolving, gave rise to four types. Among them, echinoderms, having formed a water-vascular ambulacral system and a complex apparatus for capturing food, gained the ability to move on different soils and switched to active feeding on immobile and inactive food objects. This ensured their biological success: in many biocenoses of the seabed, not only in shallow waters, but also at great depths, echinoderms flourish without serious competitors.

Pogonophores- a peculiar group of sedentary animals, now distinguished as a special type (A.V. Ivanov, 1955), continues to cause controversy regarding their origin and position in the system. Pogonophores sit in protective tubes and are distinguished by a very simplified structure: a central nervous system from the dorsal trunk with a head ganglion, the absence of organs of movement and the digestive tube. They live on nutrients dissolved in water - decomposition products of the "rain of corpses" descending from the life-rich layers of water lying above. They are characterized by the so-called extraintestinal digestion: absorption is carried out by the cells of the tentacles. Such passive feeding is possible and expedient in the weakly moving water of the ocean depths.

The third branch of development led to the isolation of chordates. Apparently, at the very beginning of evolution, a small group of hemichordate animals separated from it at the present time, which is now given the rank of type. Type hemichordates ( Hemichordata) includes two classes: pinnatibranchs ( Pterobranchia) and enteric ( Enteropneusta). Representatives of both classes have a three-segmented body, consisting of a head lobe (proboscis), a collar and a trunk.

pinnatibranch- sedentary animals forming colonies in the form of bushes; in the cavities of the tubules (twigs of the bush) animals - zooids sit. The small hollow head lobe (proboscis) of the zooid has muscular walls and communicates with the external environment by a small pore. Inside, at the base of the head lobe, there is a "heart" 1 and an excretory organ, and on its surface there is a glandular organ, the secret of which serves to build the walls of the tubules - the branches of the colony. The collar, which has its own internal cavity, frames the mouth opening and serves as a support for the branching tentacles - the respiratory and food collection organs. On the dorsal surface of the collar, a short chain of nerve ganglia is located intraepithelially, extending to the proboscis. The trunk is occupied by a curved intestinal tube.

Genus representatives Cephalodiscus and Atubaria in the upper part of the intestinal tube they have a pair of "gill" openings that open outwards, which, however, are not related to breathing and serve only to discharge water during filtration. The collar with tentacles and the base of the proboscis are supported by the notochord, a small elastic outgrowth of the dorsal part of the intestine, which allows us to consider the notochord as the rudiment (predecessor) of the notochord. In the body cavity (coelom) are the sex glands, which open outwards with short ducts. From a fertilized egg, a mobile larva, capable of crawling and swimming, develops, which soon sits on the bottom and after two days turns into an adult animal. The last way budding forms new colony. Kidneys are formed on the stolon, in the tail section of the body.

Enteric-breathing have an elongated worm-shaped body; length from a few centimeters to 2-2.5 m ( Batanoglossus gigas). They lead a solitary lifestyle, are quite mobile, live mainly in shallow sea waters, but are also found at depths up to 8100 m. In the ground, by mining, they make U-shaped mink passages. Their walls are held together by mucus secreted by glandular skin cells.

The proboscis has muscular walls; through a small hole, its cavity can be filled with water, turning the proboscis into a tool for making holes. There is also a small whole inside the collar. On the ventral side, between the proboscis and the collar, there is a mouth opening leading to the pharynx (Fig. 4). The walls of the pharynx are pierced by numerous paired gill slits that open outward on the dorsal side of the body; at the bottom of the pharynx in some species, a longitudinal thickening is formed, which can probably be considered as the rudiment of the endostyle. The pharynx passes into the intestine, ending with the anus at the posterior end of the body. Numerous blind hepatic outgrowths depart from the dorsal surface of the anterior part of the intestine; they are visible from the outside as rows of tubercles. At the base of the proboscis, as in pinnatibranchs, a small hollow elastic outgrowth of the pharyngeal wall protrudes, formed by vacuolized cells and strands of connective tissue, the notochord. In balanogloss, several muscular bands are connected with the notochord, going to the caudal part of the body. This can be seen as a prototype of that myochordial complex, with the development and improvement of which the progress of chordates is associated.

Circulatory system open. Two longitudinal vessels - dorsal and abdominal - are connected by transverse vessels passing through the partitions between the gill slits. The dorsal vessel opens into the head lacuna located above the notochord. Adjacent to it is the "heart" - a hollow muscular vesicle: its rhythmic contractions create a blood flow. A folded formation penetrated by blood vessels protrudes into the cavity of the proboscis, performing the function of an excretory organ; its epithelium is similar to the epithelium excretory organs chordate animals. The decay products diffuse into the cavity of the proboscis and are brought out with water through the proboscis pore. Breathing is carried out both by the entire surface of the body and in the pharynx: oxygen enters the blood flowing through the vessels of the interbranch septa. The nervous system consists of dorsal and abdominal nerve cords connected by one or two parapharyngeal nerve rings (commissures). In the anterior part of the dorsal nerve cord there is usually a cavity similar to the neurocoel of the neural tube of chordates. The sense organs are represented by sensory epidermal cells, which are more numerous on the proboscis and the anterior part of the collar. Sensory cells scattered at the top of the proboscis are light sensitive.

Chordates are one of the largest types of the animal kingdom, whose representatives have mastered all habitats. This type includes three groups (subtypes) of organisms: tunicates(appendiculars, ascidians, salps), non-cranial (similar to small fish sea creatures - lancelets), vertebrates. The non-cranial and tunicate subtypes are usually referred to as the lower chordates, contrasting them with the higher chordate-vertebrates. Despite the exceptional diversity of chordates, they all have a number of common structural and developmental features. The main ones are:

1. All chordates have axial skeleton, which initially appears in the form dorsal string, or chords(fig.1). The notochord is an elastic, non-segmented strand that develops embryonically by lacing it from the dorsal wall of the germinal gut: the notochord is of endodermal origin. The subsequent fate of the chord is different. It persists for life only in lower chordates (with the exception of ascidia and salps). In most representatives, the notochord is reduced to one degree or another in connection with the development of the spinal column. In higher chordates, it is an embryonic organ and in adult animals it is to some extent replaced by vertebrae, in connection with this, the axial skeleton from a continuous non-segmented cord becomes segmented

Fig.1. The structure of chordates on the example of the lancelet

2. Above the axial skeleton is located central nervous system, represented by a hollow tube. The cavity of the neural tube is called neurocoel. The tubular structure of the central nervous system is characteristic of almost all chordates. The only exceptions are adult tunicates. In almost all chordates, the anterior neural tube grows and forms the brain. The internal cavity is preserved in this case in the form of the ventricles of the brain. Embryonally, the neural tube develops from the dorsal part of the ectodermal bud.

3. The anterior (pharyngeal) section of the digestive tube communicates with the external environment by two rows of holes, called visceral fissures.. In lower forms, gills are located on their walls. Gill slits are preserved for life only in lower aquatic chordates. For the rest, they appear only as embryonic formations, functioning at some stages of development or not functioning at all. Along with the indicated three main features of the chordates, the following characteristic features of their organization should be mentioned, which, however, besides the chordates, representatives of some other groups also have.

In addition to these typical features, chordates have some features that are also found in other types.

1. By breaking through the wall of the gastrula, a secondary mouth is formed; in the region of the primary mouth (gastropore), an anus is formed. This feature combines chordates with hemichordates, echinoderms, chaetognaths and pogonophores into a group of deuterostomes - Deuterostomia, opposed to a group of protostomes - Protostomia, in which an oral opening is formed at the site of the gastropore, and the anus is formed by breaking through the wall of the gastrula (all other types of animals belong to protostomes, except for sponges, coelenterates and protozoa).

2. In the process of embryonic development, a secondary body cavity is formed - the whole, but all deuterostomes, annelids, mollusks, arthropods, bryozoans and brachiopods also possess it.

Modern ideas about the origin of chordates combine the provisions that previously existed within the framework of alternative hypotheses of the origin of chordates: the inversion of chordates, their relationship with deuterostomes, and the fact that tunicate larvae are more primitive than other chordates. Contrary to pre-existing points of view, chordates are considered not as inverted annelids and arthropods, but as inverted deuterostomes. As for the larvae of the ascidians, they are not the ancestors of the rest of the chordates, as is accepted in the hypotheses of the neotenic origin of the chordates. The organization of ascidian larvae recapitulates (that is, repeats in a compressed, modified form) in individual development a certain phylogenetic stage in the historical development of chordates, namely the structure of the ancestors of chordates before their revolution.

The discussion about the origin of chordates is not over. The development of biological science brings new arguments in favor of one or another point of view. Sometimes these arguments come from a completely unexpected direction. So, relatively recently, in 1993, an article by biologists Slack, Holland and Graham appeared in the English journal "Nature", analyzing numerous publications in which the expression zones of one interesting group of genes (the so-called Hox group) were identified in the development of various invertebrates and vertebrate animals. These publications showed that in invertebrates these genes mark the ventral side, while in vertebrates their expression is observed on the dorsal side. This fact is in good agreement with the idea of inverted chordates accepted by modern comparative anatomy.