Characteristics of the subtype tunicates. Type Chordates (Chordata)

Tunicates, larval chordates, or tunicates, which include ascidians, pyrosomes. salps and appendicularians are one of the most amazing groups of marine animals. Central location among them belongs to ascidians. The tunicates got their name due to the fact that their body is dressed on the outside with a gelatinous shell, or tunic. The tunic consists of a special substance - tunicin, extremely similar in composition to plant fiber - cellulose, which is found only in the plant kingdom and is unknown to any other group of animals. Tunicates are exclusively marine animals. Ascidians lead an attached way of life, the rest are free-floating pelagic. They can be solitary or form colonies arising from the alternation of generations as a result of the budding of asexual single individuals. Ascidians have a tailed larva that swims freely in the water.
All tunicates, with the exception of a few unusual predatory species, feed on organic particles suspended in the water (detritus) and phytoplankton and are active filter feeders. In the vast majority of cases, in adulthood, they have a bag-shaped or barrel-shaped body with two siphons - inlet and outlet. The siphons are either close together on the upper part of the body, or located at its opposite ends.

Representative of the subtype tunicates (Tunicata). Photo: Minette Layne

The position of tunicates in the system of the animal kingdom is very interesting. The nature of these animals remained mysterious and incomprehensible for a long time, although they were known to Aristotle more than two and a half thousand years ago under the name Tethya.
Only at the beginning of the 19th century it was established that the solitary and colonial forms of some tunicates - salps - represent only different generations of the same species. Prior to this, they were classified as different types animals. Solitary and colonial forms differ from each other not only in appearance. It turned out that only the colonial forms have sexual organs, and the solitary forms are asexual. The phenomenon of alternation of generations in salps was discovered by the poet and naturalist Albert Chamisso during his voyage in 1819 on the Russian warship "Rurik" under the command of Kotzebue. Old authors, including Carl Linnaeus, attributed the tunic to the type of molluscs. Colonial forms were assigned by him to a completely different group - zoophytes, and some considered them a special class of worms. But in fact, these outwardly very simple animals are not as primitive as they seem. Thanks to the work of the remarkable Russian embryologist A. O. Kovalevsky in the middle of the last century, it was established that tunicates are close to chordates. A. O. Kovalevsky established that the development of ascidians follows the same type as the development of the lancelet, which, according to the apt expression of Academician I. I. Shmalhausen, “is like a living simplified scheme of a typical chordate animal. The group of chordate animals is characterized by a number of certain important features of the structure.First of all, it will be the presence of a dorsal string, or chord, which is the internal axial skeleton of the animal.The tailed larvae of ascidians also have a chord, which disappears when they turn into an adult.The larvae and other important features of the structure are much higher than the parent forms. For phylogenetic reasons, i.e., for reasons connected with the origin of the group, more importance is attached to the organization of their larvae in tunicates than to the organization of adult forms. Such an anomaly is unknown in any other type of animal. Except for the presence of a notochord, at least in larval stage, with real chordates, tunicates are brought together by a number of other signs. It is very important that the nervous system of the tunicates is located on the dorsal side of the body and is a tube with a channel inside. The neural tube of the tunicates is formed as a groove-like longitudinal protrusion of the surface integuments of the body of the embryo - the ectoderm, as is the case in all other vertebrates and in humans. In invertebrates, the nervous system always lies on the ventral side of the body and is formed in a different way. The main vessels of the circulatory system of tunicates, on the contrary, are located on the ventral side, in contrast to what is characteristic of invertebrates. And finally, the anterior part of the intestine, or pharynx, is pierced by numerous holes in the tunicates and has become not only a digestive organ that filters food, but also a respiratory organ. As we saw above, in invertebrates the respiratory organs are very diverse, but the intestines never form gill slits. This is a character of chordates and is the only character that is preserved in adult forms of tunicates. The secondary cavity of the body, or the whole, is present in the tunicates, but is greatly reduced.
According to the ideas of A. O. Kovalevsky, accepted by many, although not all modern zoologists, ascidians originated from free-swimming chordates. Features of their structure - a secondary simplification, as a result of the second, both the chord, and the neural tube, and sensory organs are lost, as well as the presence of a tunic that performs protective and supporting functions, and greater specialization - is a consequence of adaptation to an attached lifestyle in adulthood. The structure of their complexly organized tailed larva, floating in the water, to some extent reproduces the organization of their ancestors.
The position of tunicates in the system of the animal kingdom remained unresolved for a long time. They were considered either as an independent type, close to chordates, or as a separate subtype of the chordate type. This was due to the poor knowledge, first of all, of the embryonic and ontogenetic development of this group of animals. Held in recent times a comparative study of the embryology of the lancelet and ascidians, pyrosomes, salps and appendicularia shows many similarities. And, as you know, the early stages of animal development are extremely great importance for phylogenetic constructions. It should be considered finally established that the tunicates are a special subtype - Urochordata, or Tuncata, of the chordate type (Chordata), where they are included together with the subtypes of the acrania (Acrania) and vertebrates (Vertebrata). It must be emphasized, however, that even now some questions concerning the relationship between these subtypes and within them, as well as the origin of chordates as a whole, still remain controversial.
A well-known specialist in the field of comparative embryology of lower chordates (skullans and tunicates), as well as invertebrates, O. M. Ivanova-Kazas notes that the development of tunicates, despite its extreme originality, allows us to consider them even more highly organized animals than the lancelet, which is the most primitive representative of the chordates and the type of development of which in the process of evolution led to the ontogenesis of vertebrates. The development of the tunicates has evolved in a different direction than the development of the lancelet. In connection with the sedentary lifestyle of ascidians, highly developed and specialized forms of asexual reproduction arose in tunicates, which is completely uncharacteristic of other chordates, with complex life cycles, with the emergence of coloniality, polymorphism, etc. It was inherited from ascidians by pyrosomes and salps.
Tunicates reproduce both asexually (budding) and sexually. Some of them are hermaphrodites. The reproduction of the tunicates is an amazing example of how extraordinarily complex and fantastic life cycles animals may be in nature. All tunicates, except appendicularia, are characterized by both sexual and asexual reproduction. In the first case, a new organism is formed from a fertilized egg. But in tunicates, development to an adult occurs with profound transformations in the structure of the larva towards its significant simplification. With asexual reproduction, new organisms, as it were, bud off from the mother individual, receiving from her the rudiments of all the main organs.
All sexual individuals of tunicates are hermaphrodites, that is, they possess both male and female gonads. The maturation of male and female reproductive products always occurs in different time and therefore self-fertilization is impossible. In ascidians, salps, and pyrosomes, the gonadal ducts open into the cloacal cavity, and in the appendicularia, spermatozoa enter the water through ducts that open on the dorsal side of the body, while eggs can only come out after the rupture of the body walls, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when the sperm meets the egg in the water and fertilizes it there. In salps and pyrosomes, only one egg is formed, which is fertilized and develops in the mother's body. It should be emphasized that the acquisition of mobility by pelagic tunicates led to the loss of their developed free-swimming larvae. In complex and in most solitary ascidians, the fertilization of eggs occurs in the cloacal cavity of the mother, where the spermatozoa of other individuals penetrate with the flow of water through the siphons, and the fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then go outside, that is, a kind of live birth takes place.
For sessile organisms, for their successful reproduction, it is necessary that the eggs and spermatozoa of neighboring individuals mature at the same time. This synchronization is achieved by the fact that the sexual products brought out by the first sexually mature individuals, with the flow of water, enter through the introductory siphon to neighboring animals and in a short time stimulate the beginning of their reproduction in large areas. A special role is played by the paranervous gland, which communicates with the pharyngeal cavity and receives the corresponding signal from the water. Through the nervous system, it accelerates the maturation of the gonads.
Many features of the embryonic development of the lancelet and tunicate are close to those, for example, in echinoderms or hemichordates, and this allows us to consider lower chordates as a kind of link between invertebrates and vertebrates.
However, neither acraniates nor tunicates appear to be direct ancestors of vertebrates. The origin of the tunic is currently presented as follows. Some primitive non-cranial moved to a sedentary lifestyle on a solid substrate at the bottom of the sea and turned into sea squirts. A powerful tunic protected them well from enemies, and a well-developed pharyngeal filtration apparatus provided enough food for these animals, which switched to a passive way of feeding and became filter feeders - stenophages. Part of the important organs in adult organisms was reduced in this case. They remained only in the active free-swimming larva, which allowed the immobile ascidians to spread widely in the ocean. BUT amazing ability to asexual reproduction - budding ensured the rapid settlement of new sites. Then the tunicates repopulated aquatic environment and managed to master the jet method of movement. All this gave them great advantages, but although tunicates are widespread in modern seas and oceans and are a characteristic component of marine fauna, they did not give a progressively developing branch on the evolutionary tree. This is, as it were, an evolutionary dead end, a lateral branch extending from the very base of the phylogenetic trunk of chordates.
Together with other chordates and a small number of invertebrates, tunicates belong to deuterostomes, one of the main trunks of the evolutionary tree in the kingdom Animalia.
In representatives of deuterostomes, or Deuterostomia, in the process of embryonic development, the mouth is not formed in the place of the primary mouth of the embryo, but breaks through anew. The primary mouth turns into an anus. In contrast, in protostomes, or Protostomia, the mouth is formed in place of the mouth of the embryo - the blastopore. Most of the types of invertebrates belong to them.
The subtype of tunicates includes three classes: ascidians (Ascidiae), salps (Salpae) and appendiculars (Appendiculariae). Ascidians gave rise to other classes of tunicates.
The subphylum includes 1100 marine-dwelling species. Of these, 1000 species are accounted for by ascidians. There are about 60 species of appendicularia, about 25 species of salps, and approximately 10 species of pyrosomes. The structure of the body of almost all tunicates is unrecognizably very different from the general plan of the body structure in the type of chordates.

Type Chordates combines animals, different in appearance, living conditions, lifestyle. Representatives of this type are found in all the main environments of life: in water, on land, in the thickness of the soil, in the air. They are distributed throughout the earth. The number of species of modern representatives of chordates is about 40 thousand.

The phylum Chordata includes non-cranial, cyclostomes, fish, reptiles, amphibians, mammals, and birds. Tunics can also be attributed to this type - this is a peculiar group of organisms that lives on the bottom of the ocean and leads an attached lifestyle. Sometimes included in the phylum Chordates are enteropneas, which have some of the characteristics of this type.

Characters of the chordate type

Despite the great diversity of organisms, they all have a number of common structural and developmental features.

The structure of chordates is as follows: all these animals have an axial skeleton, which first appears in the form of a chord or dorsal string. The notochord is a special non-segmented and elastic cord that embryonic develops from the dorsal wall of the embryonic intestine. The origin of the notochord is endothermal.

Further, this cord can develop in different ways, depending on the organism. For life it remains only in the lower chordates. In most higher animals, the notochord is reduced, and the vertebral column is formed in its place. That is, in higher organisms, the notochord is an embryonic organ that is displaced by the vertebrae.

Above the axial skeleton is the central nervous system, which is represented by a hollow tube. The cavity of this tube is called the neurocoel. Almost all chordates are characterized by a tubular structure of the central nervous system.

In most organisms of the chordate type, the anterior section of the tube grows to form the brain.

The pharyngeal section (anterior) of the digestive tube comes out with two opposite ends. The outgoing openings are called visceral fissures. At lower organisms type they have gills.

In addition to the above three features of chordates, it can also be noted that these organisms have a secondary mouth, like echinoderms. The body cavity in animals of this type is secondary. Chordates also have bilateral body symmetry.

The phylum Chordates is divided into subtypes:

• Skullless;
• tunicates;
• Vertebrates.

Subtype Cranial

This subtype includes only one class - the Head Chordidae, and one order - the Lancelets.

The main difference of this subtype is that these are the most primitive organisms, and all of them are exclusively marine animals. They are distributed in warm waters ah oceans and seas of temperate and subtropical latitudes. Lancelets and epigonichites live in shallow water, mainly burying themselves with the back of the body in the bottom substrate. They prefer sandy soil.

This type of organism feeds on detritus, diatoms or zooplankton. They always breed in the warm season. Fertilization is external.

The lancelet is a favorite object of study, since all the signs of chordate organisms are preserved in it for life, which makes it possible to understand the principles of the formation of chordates and vertebrates.

Subtype Shellers

The subtype includes 3 classes:

• salps;
• ascidians;
• Appendiculars.

All animals of the subtype are exclusively marine.

The main difference between these chordates is that in almost all organisms in the adult state there is no chord and neural tube. In the larval state, all type traits in tunicates are pronounced.

Tunicates live in colonies or singly, attached to the bottom. There are much fewer free-swimming species. This subtype of animals lives in the warm waters of the tropics or subtropics. They can live both on the surface of the sea and deep in the ocean.

The body shape of adult tunicates is rounded barrel-shaped. The organisms got their name due to the fact that their body is covered with a rough and thick shell - a tunic. The consistency of the tunic is cartilaginous or gelatinous, its main purpose is to protect the animal from predators.

Tunicates are hermaphrodites, they can reproduce both sexually and asexually.

It is known that the ancestors of these organisms were free-swimming, while at the present time only tunicate larvae can move freely in the water column.

Subtype Vertebrates

Skull animals are the highest subtype. Compared to other subtypes, they have more high level organizations, which can be seen from their structure, both external and internal. Among vertebrates, there are no species that lead a completely attached lifestyle - they actively move in space, looking for food and shelter, a mate for reproduction.

By moving, vertebrate organisms provide themselves with the opportunity to change their habitat depending on changing external conditions.

The above general biological features are directly related to the morphological and physiological organization of vertebrates.

The nervous system of the cranial is more differentiated than that of the lower animals of the same type. Vertebrates have a well-developed brain, which contributes to the functioning of higher nervous activity. It is the highest nervous activity is the basis adaptive behavior. These animals have well-developed sense organs, which are necessary for communication with the environment.

As a result of the emergence of the sense organs and the brain, such a protective organ as the skull has developed. And instead of a chord, this subtype of animals has a vertebral column, which performs the function of supporting the entire body and a case for the spinal cord.

All animals of the subtype develop a mobile jaw apparatus and oral fissure, which develop from the anterior intestinal tube.

The metabolism of this subtype is much more complicated than that of all the animals discussed above. Cranials have a heart that provides fast blood flow. The kidneys are essential for removing waste products from the body.

The subphylum Vertebrates appeared only in the Ordovician-Silurian, but in jurassic already existed all now known types and classes.

The total number of modern species is slightly more than 40 thousand.

Vertebrate classification

Very diverse type of chordates. The classes that exist in our time are not so numerous, but the number of species is enormous.

The cranial subtype can be divided into two groups, these are:

• Primary organisms.
• Terrestrial organisms.

Primary aquatic organisms

Primary aquatic differ in that they either have gills throughout their life, or only in the larval stage, and during the development of the egg, embryonic membranes are not formed. This includes representatives of the following groups.

Section Jawless

• Class Cyclostomes.

These are the most primitive cranial animals. They actively developed in the Silurian and Devonian; at present, their species diversity is not high.

Section Jaws

Superclass Pisces:

• Class Bony fish.
• Class Cartilaginous fish.

Superclass Quadrupeds:

• Class Amphibians.

These are the first animals in which the jaw apparatus appears. This includes all known fish and amphibians. All of them actively move in water and on land, hunt and capture food with their mouths.

Terrestrial organisms

The group of terrestrial animals includes 3 classes:

• Birds.
• Reptiles.
• Mammals.

This group is characterized by the fact that embryonic membranes are formed in animals during the development of the egg. If the species lays its eggs on the ground, then the embryonic membranes protect the embryo from external influences.

All chordates of this group live mainly on land, have internal fertilization, which indicates that these organisms are more evolutionarily developed.

They lack gills at all stages of development.

Origin of chordates

There are several hypotheses for the origin of chordates. One of them says that this type of organisms originated from the larvae of the enteropretis. Most representatives of this class lead an attached lifestyle, but their larvae are mobile. Considering the structure of the larvae, one can see the beginnings of the notochord, the neural tube and other features of the chordates.

Another theory is that the Chordata phylum is descended from the crawling, worm-like ancestors of the intestinal-breathers. They had the beginnings of a chord, and in the pharynx, next to the gill slits, there was an endostyle - an organ that contributed to the secretion of mucus and catching food from the water column.

The article considered the general characteristics of the type. Chordates are united by many similar features of all organisms, but still each class and each species has individual characteristics.

Type Chordates

Inferior chordates. Subtype Cranial

TYPE CHORDS. LOWER CHORDS

general characteristics type Chordates

Type Chordates combines animals diverse in appearance and lifestyle. Chordates are distributed throughout the world, have mastered a variety of habitats. However, all representatives of the type have the following common organizational features:

1. Bilaterally symmetrical chordates, deuterostomes, multicellular animals.

2. Chordates have a notochord throughout their life or at one of the phases of development. Chord- This is an elastic rod located on the dorsal side of the body and performing a supporting function.

3. Above the chord is located nervous system in the form of a hollow tube. In higher chordates, the neural tube is differentiated into the spinal cord and brain.

4. Under the chord is located digestive tube. The alimentary canal begins mouth and ends anus, or the digestive system opens into the cloaca. Throat pierced gill slits, which in primary aquatic organisms persist throughout their lives, while in terrestrial ones they are laid only at the early stages of embryonic development.

5. Beneath the digestive system lies heart. Circulatory system in chordates closed.

6. Chordates have secondary body cavity.

7. Chordates are segmented animals. Location of organs metameric, i.e. the main organ systems are located in each segment. In higher chordates, metamerism is manifested in the structure of the spinal column, in the muscles of the abdominal wall of the body.

8. The organs of excretion in chordates are diverse.

9. Chordates have separate sexes. Fertilization and development are varied.

10. Chordates descended through a series of intermediate forms unknown to biology from the very first coelomic animals.

The chordate type is divided into three subtypes:

1. Subtype Cranial. These are 30-35 species of small marine chordates, resembling fish in shape, but without limbs. The notochord in the Skullless persists throughout life. Nervous system in the form of a hollow tube. The pharynx has gill slits for breathing. Representatives - Lancelets.

2. Subtype Larval-chordaceae, or Shellers. These are 1500 species of marine sedentary sedentary animals living in tropical and subtropical regions. Their body is in the form of a bag (the size of the body in one individual in the colony is no more than 1 mm, and single ones can reach 60 cm), there are two siphons on the body - oral and cloacal. Larval chordates are water filterers. The body is covered with a thick shell - a tunic (hence the name of the subtype - Tunics). As adults, the tunicates lack the notochord and neural tube. However, the larva, which actively swims and serves for settling, has a structure typical of Chordates and is similar to the Lancelet (hence the second name - Larval Chordates). Representative - Ascidia.

3. Subtype Vertebrates, or cranial. These are the most highly organized chordates. Nutrition in vertebrates is active: food is searched for and pursued.

The notochord is replaced by the vertebral column. The neural tube is differentiated into the spinal cord and brain. The skull is developed, which protects the brain. The skull bears jaws with teeth for grasping and grinding food. Paired limbs and their belts appear. Cranials have a much higher level of metabolism, a complex population organization, diverse behavior, and a pronounced individuality of individuals.

The subtypes Cranial and Larval Chordates are called the lower Chordates, and the Vertebrate subtype is the higher Chordates.

Subtype Cranial - Acrania

Lancelet

The subtype Cranial includes the only class of the Head Chordidae, which has only about 30-35 species of marine animals living in shallow water. A typical representative is Lancelet — Branchiostoma lanceolatum(Lancelet genus, class Headochord, subtype Cranial, type Chordata), the size of which reaches 8 cm. The body of the Lancelet is oval in shape, narrowed towards the tail, laterally compressed. Outwardly, the Lancelet resembles a small fish. Located on the back of the body tail fin in the form of a lancet - an ancient surgical instrument (hence the name Lancelet). Paired fins are absent. There is a small dorsal. On the sides of the body from the ventral side hang two metapleural folds, which fuse on the ventral side and form peribranchial, or the atrial cavity, which communicates with the pharyngeal fissures and opens at the posterior end of the body with a hole - atriopore- outside. At the anterior end of the body near the mouth are the perioral tentacles, with which the Lancelet captures food. Lancelets live on sandy soils in the sea at a depth of 50-100 cm in temperate and warm waters. They feed on bottom sediments, marine ciliates and rhizopods, eggs and larvae of small marine crustaceans, diatoms, burrowing into the sand and exposing the front end of the body. More active at dusk, avoid bright lighting. Disturbed Lancelets swim quite quickly from place to place.

Covers. The body of the lancelet is covered skin, consisting of a single layer epidermis and thin layer dermis.

Musculoskeletal system. A chord stretches along the entire body. Chord- this is an elastic rod located on the dorsal side of the body and performing a supporting function. To the anterior and posterior ends of the body, the chord becomes thinner. The notochord protrudes into the anterior part of the body a little further than the neural tube, hence the name of the class - Cephalic. The notochord is surrounded by connective tissue, which simultaneously forms supporting elements for the dorsal fin and divides the muscle layers into segments using connective tissue

Type Chordates subtype Cranial Lancelet

layers. Individual muscle segments are called myomers, and the partitions between them myoseptami. Muscles are formed by striated muscles.

body cavity at the lancelet secondary in other words, they are coelomic animals.

Digestive system. On the front of the body is mouth opening, surrounded by tentacles(up to 20 pairs). The mouth opening leads to a large throat, which functions as a filtering apparatus. Through the cracks in the pharynx, water enters the atrial cavity, and food particles are directed to the bottom of the pharynx, where endostyle- a groove with a ciliated epithelium that drives food particles into the intestine. no stomach, but hepatic outgrowth, homologous to the liver of vertebrates. midgut, without making loops, opens anus at the base of the tail fin. Digestion of food occurs in the intestines and in the hollow hepatic outgrowth, which is directed towards the head end of the body. Interestingly, the Lancelet retained intracellular digestion, intestinal cells capture food particles and digest them in their digestive vacuoles. This mode of digestion is not found in vertebrates.

Respiratory system. There are more than 100 pairs in the throat of the Lancelet gill slits leading to peribranchial cavity. The walls of the gill slits are penetrated by a dense network of blood vessels in which gas exchange occurs. With the help of the ciliary epithelium of the pharynx, water is pumped through the gill slits into the peribranchial cavity and through the opening (atriopore) is brought out. In addition, gas-permeable skin also takes part in gas exchange.

Circulatory system. The circulatory system of the Lancelet closed. The blood is colorless and contains no respiratory pigments. The transport of gases is carried out as a result of their dissolution in the blood plasma. In the circulatory system one circle circulation. The heart is absent, and blood is moved by the pulsation of the gill arteries, which pump blood through the vessels in the gill slits. Arterial blood enters dorsal aorta, from which carotid arteries blood flows to the front, and through the unpaired dorsal aorta to the back of the body. Then by veins blood returns to venous sinus and by abdominal aorta heading for the gills. All blood from the digestive system enters the hepatic outgrowth, then into the venous sinus. The liver outgrowth, like the liver, neutralizes toxic substances that have entered the bloodstream from the intestines, and, in addition, performs other functions of the liver.

Such a structure of the circulatory system does not fundamentally differ from the circulatory system of vertebrates and can be considered as its prototype.

excretory system. The excretory organs of the lancelet are called nephridia and resemble excretory organs flatworms- protonephridia. Numerous nephridia (about a hundred pairs, one for two gill slits), located in the pharynx, are tubules that open with one hole into the coelom cavity, the other - into the paragillary cavity. On the walls of the nephridium are club-shaped cells - solenocytes, each of which has a narrow channel with a ciliated hair. Due to the beating of these

Type Chordates subtype Cranial Lancelet

hairs, the liquid with metabolic products is removed from the cavity of the nephridium into the peribranchial cavity, and from there it is already out.

central nervous system formed neural tube with a cavity inside. The lancelet does not have a pronounced brain. In the walls of the neural tube, along its axis, there are light-sensitive organs - eyes Hesse. Each of them consists of two cells - photosensitive and pigmented, they are able to perceive the intensity of light. An organ adjacent to the expanded anterior part of the neural tube smell.

Reproduction and development. The lancelets that live in our Black Sea and the lancelets that live in the waters of the Atlantic off the coast of Europe break into breeding in the spring and spawn eggs until August. Warm water lancelets breed all year round. lancelets separate sexes, sex glands (gonads, up to 26 pairs) are located in the body cavity in the pharynx. Sexual products are excreted into the peribranchial cavity through the temporarily formed genital ducts. Fertilization external in water. emerges from the zygote larva. The larva is small: 3-5 mm. The larva actively moves with the help of cilia that cover the entire body, and due to the lateral bends of the body. The larva swims in the water column for about three months, then passes to life at the bottom. Lancelets live up to 4 years. Sexual maturity is reached by two years.

Significance in nature and for man. Skullless are an element biodiversity on the ground. They feed on fish and crustaceans. The Skullless themselves recycle the dead organic matter, being reducers in the structure marine ecosystems. The non-cranial are essentially a living blueprint for the structure of chordate animals. However, they are not direct ancestors of vertebrates. In the countries of Southeast Asia locals Lancelets are collected by sifting sand through a special sieve and eaten.

Non-cranial animals have retained a number of features characteristic of their invertebrate ancestors:

§ excretory system of nephridial type;

§ the absence of differentiated sections in the digestive system and the preservation of intracellular digestion;

§ filtering method of nutrition with the formation of a near-gill cavity to protect the gill slits from clogging;

§ metamerism (repetitive arrangement) of the genital organs and nephridia;

§ absence of a heart in the circulatory system;

§ weak development of the epidermis, it is single-layer, like in invertebrates.

Type Chordates subtype Cranial Lancelet

Rice. The structure of the lancelet.

A - neural tube, chord and digestive system; B - circulatory system.

1 - chord; 2. - neural tube; 3 - oral cavity; 4 - gill slits in the pharynx; 5 - peribranchial cavity (atrial cavity); 6 - atriopore; 7 - hepatic outgrowth; 8 - gut; 9 - anus; 10 - subintestinal vein; 11 - capillaries of the portal system of the hepatic outgrowth; 12 - abdominal aorta; 13 - pulsating bulbs of the arteries pumping blood through the gill slits; 14 - dorsal aorta.

Rice. Nephridium Lancelet.

1 - hole as a whole (into the secondary cavity of the body); 2 - solenocytes; 3 - opening into the circumbranchial cavity.

Type Chordates subtype Cranial Lancelet

Rice. Cross section of the Lancelet:

A - in the region of the pharynx, B - in the region of the midgut.

1 - neural tube; 2 - muscles; 3 - roots of the dorsal aorta; 4 - ovary; 5 - endostyle; 6 - abdominal aorta; 7 - metapleural folds; 8 - peribranchial (atrial) cavity; 9 - gill slits (due to the oblique position on one cross section more than one pair is visible); 10 - nephridia; 11 - whole; 12 - ventral (motor) spinal nerve; 13 - dorsal (mixed) nerve; 14 - chord; 15 - subintestinal vein; 16 - dorsal aorta; 17 - dorsal fin.

Questions for self-control.

Name the characteristic features of animals of the Chordata type.

Name the type classification into three subtypes.

Name the systematic position of the Lancelet.

Where does the lancelet live?

What is the body structure of the Lancelet?

How does the Lancelet eat and what is the structure of the digestive system of the Lancelet?

How is the excretion of waste products from the Lancelet?

What is the structure of the nervous system of the Lancelet?

What is the structure of the circulatory system of the Lancelet?

How does the lancelet reproduce?

What is the significance of the Lancelet in nature?

PICTURES TO BE COMPLETED IN THE ALBUM

(total 3 drawings)

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chordates

Chordates are the highest phylum of deuterostomes. All species of this type are characterized at least at the stage of embryonic development by the presence of an unsegmented dorsal skeletal axis (chord), dorsal neural tube, and gill slits.

Type Chordata. General characteristics. Structural features

The type is divided into three subtypes: tunicates, non-cranial and vertebrates.

Tunicates (Tunicata) or larval-chords (Urochordata) have a bag-shaped or barrel-shaped body from 0.3 to 50 cm long; the size of a colony of pyrosomes can exceed 30 m. The body of the tunicates is enclosed in a gelatinous tunic secreted by the outer epithelium.

The pharynx is pierced by gill slits. The hindgut and gonadal ducts open into the atrial cavity, which connects with external environment. The nervous system consists of a ganglion located between the mouth and the atriopore, with a nerve trunk extending from it; sense organs are poorly developed.

Tunicates reproduce sexually; asexual reproduction also occurs. All larvae are marine animals that feed on algae, small animals and detritus.

In contrast to the simplified structure of adult forms, leading sedentary image life, the larvae are active, have developed sensory organs and the nervous system, musculature and notochord (in adult forms, it remains only in appendicularia). Vertebrates are believed to have descended from neotenic (starting to breed) tunicate larvae. Three classes: tiny primitive appendiculars (Appendicularia), sea squirts (Ascidiacea) and pelagic tunicates (Thaliacea), including three subclasses: pyrosomes, salps and casks.

About 3000 species, mainly in the upper layers of the seas and oceans.

Cranial (Acrania) or cephalochord (Cephalochordata) - a subtype of lower chordates.

The head is not isolated, the skull is absent (hence the name). The entire body, including some internal organs, is segmented. Respiratory organs - gills. Blood moves due to the pulsating abdominal vessel. The sense organs are represented only by feeling cells.

The subtype includes two families (about 20 species), whose representatives live in temperate and warm seas; the most famous is the lancelet.

Vertebrates (Vertebrata) or cranial (Craniota) are the most highly organized group of animals.

Vertebrates lose, for example, to insects in terms of the number of species, but they have a very importance for the modern biosphere, as they usually complete all food chains.

Due to the presence of a complex nervous system and the ability to live in a wide variety of conditions, vertebrates were divided into sharply different systematic groups and managed to achieve not only high perfection in morphology, physiology and biochemistry, but also the ability to higher forms of behavior and mental activity.

The main features of vertebrates: the presence of a notochord in an embryo, which in an adult animal transforms into a spine, an internal skeleton, a separate head with a developed brain, a protected skull, perfect sensory organs, developed circulatory, digestive, respiratory, excretory and reproductive systems.

Vertebrates reproduce exclusively sexually; most of them are dioecious, but some fish are hermaphrodites.

The first vertebrates appeared in the Cambrian. 8 classes combined into 2 superclasses: jawless (Agnatha) - scutellous and cyclostomes and jawed (Gnathostomata) - armored, cartilaginous and bone fish, amphibians, reptiles, birds, mammals. Shield fish, as well as armored fish, became extinct in the Paleozoic. About 50,000 species of vertebrates are currently known.

General characteristics of the chordate type

The main terms and concepts tested in the examination paper: non-cranial, gill slits, internal skeleton, amphibians, skin, limbs and girdle of limbs, circulation, lancelet, mammals, neural tube, vertebrates, reptiles, birds, reflexes, lifestyle adaptations, fish, bone skeleton, cartilaginous skeleton, notochord .

To type Chordates include animals that have an internal axial skeleton - a chord or a vertebral column.

Chordate animals have reached in the process of evolution the highest, in comparison with other types, the level of organization and flourishing. They live in all areas the globe and occupy all habitats.

chordates are bilaterally symmetrical animals with a secondary body cavity and a secondary mouth.

In chordates, there is a general plan for the structure and location of internal organs:

- the neural tube is located above the axial skeleton;

- under it is a chord;

- under the chord is the digestive tract;

- under the digestive tract - the heart.

In the phylum Chordates, two subtypes are distinguished - Cranial and Vertebrate.

Refers to the non-skull lancelet. All other chordates known today, considered in school course biology, belong to the subtype Vertebrates.

The subtype Vertebrates includes the following classes of animals: Fish, Amphibians, Reptiles, Birds, Mammals.

General characteristics of chordates.Skin vertebrates protect the body from mechanical damage and other environmental influences.

The skin is involved in gas exchange and excretion of decay products.

Derivatives of the skin are hair, claws, nails, feathers, hooves, scales, horns, needles, etc. Sebaceous and sweat glands develop in the epidermis.

Skeleton, representatives of the chordate type can be connective tissue, cartilaginous and bone. The non-cranial have a connective tissue skeleton. In vertebrates - cartilaginous, bone-cartilaginous and bone.

musculature- divided into striated and smooth.

The striated muscles are called skeletal. Smooth muscles form the muscular system of the jaw apparatus, intestines, stomach and other internal organs. The skeletal muscles are segmented, although less than in lower vertebrates. Smooth muscle has no segmentation.

Digestive system presented oral cavity, pharynx, always associated with the respiratory organs, esophagus, stomach, small and large intestines, digestive glands - the liver and pancreas, which develop from the wall of the anterior intestine.

In the process of evolution of chordates, the length of the digestive tract increases, it becomes more differentiated into sections.

Respiratory system formed by gills (in fish, amphibian larvae) or lungs (in terrestrial vertebrates).

The skin serves as an additional respiratory organ for many. The gill apparatus communicates with the pharynx. In fish and some other animals, it is formed by the gill arches, on which the gill filaments are located.

The lungs during embryonic development are formed from outgrowths of the intestine and are of endodermal origin.

The circulatory system is closed. The heart consists of two, three or four chambers. Blood enters the atria, and is sent to the bloodstream by the ventricles.

There is one circulation circle (in fish and amphibian larvae) or two (in all other classes). The heart of fish, amphibian larvae is two-chambered. Adult amphibians and reptiles have a three-chambered heart. However, reptiles develop an incomplete interventricular septum. Fish, amphibians and reptiles are cold-blooded animals.

Birds and mammals have a four-chambered heart. These are warm-blooded animals.

Blood vessels are divided into arteries, veins and capillaries.

Nervous system ectodermal origin. It is laid in the form of a hollow tube on the dorsal side of the embryo. The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is made up of cranial and spinal nerves and interconnected ganglia along the spinal column.

Spinal cord is a long cord lying in the spinal canal. Spinal nerves branch off from the spinal cord.

sense organs well developed. Primitive aquatic animals have organs sideline, perceiving pressure, direction of movement, speed of water flow.

excretory organs all vertebrates are represented by kidneys. The structure and mechanism of functioning of the kidneys changes in the process of evolution.

Reproductive organs. Vertebrates are dioecious.

The sex glands are paired and develop from the mesoderm. The genital ducts are connected with the excretory organs.

Superclass Pisces

Fish appeared in the Silurian - Devonian from jawless ancestors.

There are about 20,000 species. Modern fish are divided into two classes - cartilaginous and Bone. Cartilaginous fish include sharks and rays, characterized by a cartilaginous skeleton, the presence of gill slits, and the absence of a swim bladder.

Characteristics of the chordate type (Chordata)

Bony fish include animals that have bony scales, a bone skeleton, gill slits covered with a gill cover. The appearance of fish is due to the following aromorphoses :

- the appearance of a cartilaginous or bone spine and a skull that covers the spinal cord and brain from all sides;

- the appearance of the jaws;

- the appearance of paired limbs - ventral and pectoral fins.

All fish live in water, have a streamlined body, divided into a head, body and tail.

The sense organs are well developed - sight, smell, hearing, taste, organs of the lateral line, balance. The skin is two-layered, thin, mucous, covered with scales. The muscles are almost undifferentiated, with the exception of the muscles of the jaws and the muscles attached to the gill covers. bony fish.

Digestive system well differentiated into departments.

There is a liver with a gallbladder and a pancreas. Many have developed teeth.

Respiratory organs fish have gills, and lungfish have gills and lungs. An additional function of breathing is performed by the swim bladder in bony fish. It also performs a hydrostatic function.

Circulatory system closed. One circle of blood circulation. The heart consists of an atrium and a ventricle.

Venous blood from the heart through the afferent branchial arteries enters the gills, where the blood is saturated with oxygen. Arterial blood flows through the efferent branchial arteries into the dorsal aorta, which supplies blood to the internal organs.

Fish have a portal system of the liver and kidneys, which cleans the blood of harmful substances. Fish are cold-blooded animals.

excretory system represented by ribbon-like primary kidneys. Urine flows through the ureters to the bladder. In males, the ureter is also the vas deferens.

Females have an independent excretory opening.

gonads represented by paired testes in males and ovaries in females. Many fish show sexual dimorphism. Males brighter than females attract them with their appearance, mating dances.

In the nervous system the development of the diencephalon and midbrain should be noted.

Most fish have a well-developed cerebellum, which is responsible for coordinating movements and maintaining balance. The forebrain is less developed than in the higher classes of animals.

Eyes have a flat cornea, a spherical lens.

hearing organs represented by the inner ear - the membranous labyrinth. There are three semicircular canals.

They contain lime stones. Fish make and pick up sounds.

sense organs represented by sensitive cells scattered throughout the body.

Lateral line perceives the direction of flow and water pressure, the presence of obstacles, sound vibrations.

taste cells are in the oral cavity.

The value of fish in nature and human life. Consumers of plant biomass, consumers of the second and third orders; sources food products, fats, vitamins.

EXAMPLES OF TASKS

Part A

The non-skull animals are

3) lancelet

4) octopus

A2. The main feature of chordates is

1) closed circulatory system

2) internal axial skeleton

3) gill breathing

4) striated muscles

A3. Skeleton have

1) white shark 3) stingray

2) katrana 4) piranhas

A4. Warm-blooded animals include

1) whale 2) sturgeon 3) crocodile 4) toad

There are bony gill covers

1) dolphin 3) tuna

2) sperm whale 4) electric stingray

Have a four-chambered heart

1) turtles 2) pigeons 3) perches 4) toads

1) single chamber heart and two circles of blood circulation

2) two-chambered heart and one circle of blood circulation

3) three-chambered heart and one circle of blood circulation

4) two-chambered heart and two circles of blood circulation

A8. Cold-blooded animals are

1) beaver 3) squid

2) sperm whale 4) otter

The coordination of fish movements is regulated

1) forebrain 3) spinal cord

2) midbrain 4) cerebellum

A10. No swim bladder

1) katrans 2) pike 3) perch 4) sturgeon

Part B

IN 1. Choose the right statements

1) fish have a three-chambered heart

2) the transition of the head to the trunk in fish is clearly visible

3) there are nerve endings in the organs of the lateral line of fish

4) the chord in some fish lasts a lifetime

5) fish are not capable of forming conditioned reflexes

6) the nervous system of fish consists of the brain, spinal cord and peripheral nerves

Select the features related to non-cranial animals

1) the brain is not differentiated into sections

2) the internal skeleton is represented by a chord

3) excretory organs - kidneys

4) the circulatory system is not closed

5) the organs of vision and hearing are well developed

6) the pharynx is pierced by gill slits

VZ. Establish a correspondence between the signs of animals and the type to which these animals belong.

Part C

Where can they store oxygen? deep sea fish? Why do they need to do this?

C2. Read the text carefully. Indicate the numbers of sentences in which errors were made. Explain and correct them.

1. Type of chordates - one of the largest in terms of the number of species in the animal kingdom. 2. The internal axial skeleton in all representatives of this type is the chord - a bone, dense, elastic strand 3. The Chordata type is divided into two subtypes - Vertebrates and Invertebrates.

4. In the nervous system greatest development receives the anterior part of the brain. 5. All chordates have radial symmetry, a secondary body cavity, and a closed circulatory system. 6. An example of primitive chordates is the lancelet.

tunicates (larval chordates; Tunicata or Urochordata), a subtype of chordates, includes three classes (ascidia , Appendicularia and salps), uniting 1100-2000 species. These are widespread sedentary marine organisms, the body of which is enclosed in a shell secreted by the outer epithelium - a tunic (hence the name). The body length is from 0.3 cm to 30 m. Only larval forms have a notochord. Some lead an attached lifestyle and are single forms or branching colonies. Others swim slowly in the water column. The most noticeable organ of the tunicates is the anterior part of the U-shaped digestive tract - the pharynx, which occupies most of the volume of the body. Food is provided by filtration. They prey on small unicellular animals and plants and small organic remains. The circulatory system of the tunicates is open, lacunar type, consists of a heart sac and a developed network of lacunae. Blood moves through large vessels, and then pours into the cavities that wash the organs. The nervous system is represented by the brain ganglion on the dorsal side of the body and the nerve trunk extending from it. Tunicates are hermaphrodites, many of them are capable of asexual reproduction by budding. Ascidian class ( ascidiae) . The majority of tunicates, represented by sessile forms, both solitary and colonial, belong to this class. Colonial forms sometimes lead a free-floating lifestyle. Ascidia looks like a two-necked jar. With the base of her body (sole), she is attached to the protrusions of the bottom. On the upper part of the body there is a tubular outgrowth with an opening leading to a huge sac-like pharynx. This is the oral siphon. Another opening is located lower on the side - this is the cloacal siphon. The pharynx is pierced by a large number of small openings - gill slits, or stigmas, through which water circulates. At the bottom of the pharynx is an opening leading to a short esophagus. The esophagus passes into the sac-like stomach. The short intestine opens into the atrial cavity, which communicates with the external environment through an opening - the atriopore, located on the cloacal siphon. Power is passive. There is an endostyle. Food particles that have fallen into the throat with water are deposited on it. The endostyle begins at the bottom of the pharynx and rises along its ventral side to the mouth opening. Here it bifurcates, forming a peripharyngeal ring, and passes into a dorsal outgrowth stretching along the dorsal side of the pharynx. Food boluses are distilled by the ciliated cells of the endostyle up to the peripharyngeal ring, from where they descend along the dorsal outgrowth to the esophagus. There is a stomach, a short intestine opens into the atrial cavity near the cloacal siphon. The circulatory system is open, lacunar. The nervous system consists of a ganglion devoid of an internal cavity, located between the oral and cloacal siphons. There are no sense organs. reproductive system. Ascidians are hermaphrodites: in the body of one individual there is both an ovary and a testis. At asexual reproduction on the ventral side of the body of the mother individual, a flask-shaped protrusion appears - a kidney-shaped stolon. The kidney soon separates and turns into a sessile form: in colonial ascidians, the kidney remains on the stolon and itself begins to multiply by budding. All organs of the maternal form are formed in the kidneys. sexual reproduction sea ​​squirts: from a fertilized egg, a free-swimming larva quickly forms. Outwardly, it resembles a tadpole: its "head" contains all the organs, and the tail allows you to move quickly. In the tail, in addition to the muscles and the fin fold, a chord and a neural tube are laid. Soon it is attached by two outgrowths of the head to the substrate and undergoes a regressive metamorphosis. The chord disappears. Decrease in size, and then the neural tube, photosensitive eye and cerebral vesicle disappear. Only the posterior thickened part of the vesicle remains, which forms the ganglion. The pharynx grows, the number of gill openings increases sharply. The mouth and anus move upward. The body takes on a bag-like appearance typical of an adult. A tunic quickly forms on the surface of the body. The tunicates had common ancestors. The ancestors of the tunicates were free-swimming animals moving in the water with the help of a long tail fin. They had a developed neural tube with an expanded brain bladder at the anterior end, sensory organs in the form of an auditory vesicle and a pigmented eye, and a well-developed chord. Later, most species switched to a sedentary lifestyle and the structure of their body was greatly simplified. Progressively developed adaptations due to a sedentary lifestyle: a thick tunic - reliable protection for internal organs, a complex gill apparatus, endostyle, reproduction not only sexually, but also by budding.

hullers They represent a sharply isolated group, differing in their organization and way of life. These are marine solitary or colonial animals, leading sessile (attached to the substrate) or sedentary image life.

Typical signs of chordates are clearly expressed only on larval stage. The body shape is sac-shaped or barrel-shaped. Outside, the body is covered with a special shell - tunic containing fiber-like substance - tunicin(this is the only case in the animal world of the formation of a substance close to plant fiber).

The notochord is present only in the larval state, with the exception of appendicularia, in which the remnants of the notochord persist for life. There is no tubular nervous system in adult tunicates. They feed passively, filtering large masses of water. The circulatory system is open, lacunar type.

They reproduce both sexually (tunicates are hermaphrodites) and asexually (by budding).

The subtype includes three classes: Ascidia (Ascidiae), Salp (Salpae) and Appendicular (Appendiculariae).

Ascidian class includes about 1 thousand species of single or colonial marine animals. Most adults lead a sedentary lifestyle; larvae are free-living. Outwardly resemble double jar, attached by the base to the substrate and having two holes in the upper part of the body - oral and cloacal siphons.

Outside, the body is covered with a tunic secreted by the epithelium, which is saturated with inorganic salts, which turns it into a dense protective shell.

Under the tunic lies skin-muscular sac or mantle. Water is pumped into the pharynx by contraction and relaxation of the mantle muscles, as well as by the flickering of the cilia of the epithelium of the inner walls of the oral siphon.

Siphons have special annular bundles of muscles that close and open these openings.

The pharynx of ascidians occupies a large part of the body, its walls are pierced by many holes-stigmas that open into a special circumbranchial cavity that encloses the pharynx. Ascidians, like lancelets, have an endostyle in the pharynx, the mucus of which traps food particles from water entering through the oral siphon. Power is passive (filtered). Food particles enter the esophagus, then into the stomach, where digestion and absorption take place, undigested residues through the anus with a stream of water are removed from the body through the cloacal siphon. The pharynx also serves as a respiratory organ, gas exchange occurs in the vessels braiding the pharynx.

The heart looks like a short tube and is located on the ventral side of the body near the stomach. From the anterior end of the heart, a vessel departs, carrying blood to the walls of the pharynx. The vessel extending from the posterior end branches and approaches the internal organs (stomach, intestines, genitals) and the mantle, where it flows into small gaps located between the organs. Circulatory system open. The heart pulsates so that the blood rushes out of it alternately either towards the pharynx, where it is saturated with oxygen, or in the opposite direction. Thus, in ascidians, the same vessels are either arteries or veins.

The nervous system of an adult ascidian is represented by nerve ganglion(devoid of an internal cavity), located near the oral siphon.

All sea squirts - hermaphrodites. The sex glands are located near the stomach. The ducts of the glands flow into the peribranchial cavity. Sexual products through the cloacal siphon are excreted into the environment. Fertilization occurs either in the peribranchial cavity, where the reproductive products of another individual enter with a current of water, or in the external environment. Self-fertilization does not occur, since eggs and sperm mature in one individual at different times. Ascidians reproduce both sexually and asexually. The embryonic development of ascidians is of great general biological interest, since knowledge of it made it possible to establish the true position of the tunicates in the animal system and the undoubted belonging to chordates, because it is the larva that has all the typical signs of chordates.

In the process of development of a fertilized egg, a tailed larva is formed, outwardly similar to a tadpole, it leads a free-swimming lifestyle and sharply differs in structure from adult ascidians. It has an oval body and a long tail. The nervous system is represented by the neural tube, which has an extension of the neurocoel in the head section - the cerebral vesicle,

Where are the pigmented eye and statocyst located? The larva has a notochord - an elastic cord of highly vacuolated cells, located, like in all typical chordates, under the neural tube. Muscle cells lie on the sides of the notochord.

Asexual reproduction is carried out by budding.

Salpa class - free-swimming marine animals, in the world fauna there are about 25 species. The sizes of single individuals are from a few millimeters to 5-15 cm. The length of polymorphic colonies of barrel boars can reach 30-40 cm. They have structural features in common with ascidians, but differ in their ability to jet propulsion. The body resembles a barrel, oral and cloacal siphons are located at opposite ends of the body. The tunic is thin, transparent, so that the internal organs are clearly visible through it.

The mantle is formed by a single-layer epithelium: the musculature is arranged in the form tapes which, like hoops, encircle the body salps. With the successive contraction of the muscle bands, water is pushed out of the cloacal siphon and informs the animal forward movement. In the adult state, there is no notochord. Salps are characterized by alternation of sexual and asexual generations (metagenesis). Fertilized eggs produce asexual salps that reproduce by budding. Budding individuals form gonads and reproduce sexually. There are no free-swimming larvae characteristic of ascidians.

Appendicular class unites about 60 species of small tunicates with a body length of several millimeters; only some species reach a length of 1-2 cm. Appendicularia lead a free-floating lifestyle. Representatives of this class are the least in comparison with other Shellers evade typical chordates.

Appearance and internal structure remind ascidian larvae, differing only in details. Appendicularia have an oval body with a long, compressed tail. Throughout their lives they have chord, covered with connective tissue. The chord runs from the base to the end of the tail. Above the chord lies the nerve trunk, and on the sides - two muscle cords

The nervous system consists of the nerve ganglion, from which the nerve trunk departs, passing along the dorsal side of the tail.

There are statocysts. Gill openings two. There is no peritoneal cavity.

On the ventral side of the body lies a small heart, making up to 250 contractions per minute.

There is no real tunic in appendiculars. The animal is surrounded by a gelatinous "house", from which the appendicularia emerges several times a day, destroying its walls with its tail. The front of the house has a hole covered with a grate of thickened filaments of slime. Inside the house is located trapping net» from thin elongated formations, the mouth of the animal is turned to its top. The “house” of the appendicularium is formed by products of the secretion of the skin epithelium containing chitin-like substances.

They reproduce only sexually, without a distinct larval stage.