Tunicates or tunicates are the most interesting and amazing animals that inhabit the expanses of the ocean. They meet like seabed, as well as in the water column. This group got its name because of the outer shell, which includes a substance similar to plant fiber.

Different types of tunicates are distinguished by a wide variety of forms, and in terms of their way of life they are not similar to any species living in the world's oceans.

Shellers and their classification

The chordate phylum is divided into three subtypes, one of which is the tunicate or larval subtype. It is divided into three classes: ascidians (lat. Ascidiae), appendiculars (lat. Appendiculariae) and salps (lat. Salpae). It is believed that ascidians are the most ancient creatures that are the ancestors of representatives of other classes.

All these animals are inhabitants of the sea. They are divided into more than 1100 species. The class of ascidians is the most numerous, it includes more than 1000 species, and 10 are free-floating pyrosomes. Appendicularia account for about 60 species, salps - more than 25.

Features of the external and internal structure

The body of most tunicates (tunicates) is shaped like a bag with two holes. Through them in oral siphon water enters, then passing through the volumetric pharynx. Here it passes through numerous gill openings and enters the cloacal siphon, through which it is brought out.

Food particles are retained by the mucous membrane of the pharynx and sent through the esophagus to the stomach and then to the intestines. Undigested residues are excreted into the cloacal siphon. Thus, the adult individual has a very simple structure.

The body of the salpa resembles an ascidian. These are typical planktonic organisms that form complex colonies. Outwardly, appendicularia differ from most representatives of the tunicates subtype.

Mature individuals have almost the same organs as their larvae. They do not lose their tail, chord and balance organs all their lives. They live in a chitinous house built by them. If necessary, the appendicular leaves the house, and then creates a new one.

Tunicates or tunicates belong to the type of chordates, however, adult animals do not have chords. Only their larvae, which look like tadpoles, can be called typical representatives of this type.

They have an elongated body and tail, with the help of which they move freely in the water column. Tunicates at the larval stage of development have a much more complex structure than adults.

Young tunicates have a sensory organ, represented by a light-sensitive eye and nervous system with the rudiments of the brain. In an adult animal, the larva is reborn very quickly. Her tail retracts and disappears in a matter of hours. She loses her sense organ, brain and chord within a few days.

However, tunicates still have many signs of chordates. So their nervous system is located on the dorsal side of the body. In this they differ from invertebrates, in which it is on the ventral side.

The circulatory system, on the contrary, is located on the ventral side, while in invertebrates it is always located on the side of the back. The respiratory organs of invertebrates are very diverse, but gill slits are never formed as part of the digestive system. This feature is characteristic only for chordates.

Reproduction of tunicates

The methods of reproduction of different classes of tunicates are not the same. Ascidians reproduce both asexually and sexually. When budding, a young ascidia grows on the body of the mother organism. The sexual path involves the fertilization of eggs by sperm.

These creatures, like the rest of the tunicates, are hermaphrodites, female and male germ cells develop in the same organism, but mature in different time which prevents self-fertilization.

A tailed larva emerges from a fertilized egg, which can swim many kilometers in search of a suitable substrate. Attached to him, she is reborn into an adult.

Salp colony

Pyrosomas, which are floating colonial animals, do not have a larval stage. Their young body resembles a mature individual that makes up the colony.

Appendicularia reproduce only sexually. The eggs mature in the parent's body. When the eggs are released, the adult appendicular dies.

The reproduction and development of salps, which are part of the tunicate subtype, is very complex and interesting. They have both sexual and asexual reproduction, but alternation of these methods in different generations of the same species has been noted.

as typical representatives marine fauna, tunicates are widespread in all seas and oceans. However, many scientists do not consider this branch of evolution to be developing and progressive. Experts call the larval chordates an evolutionary dead end. These creatures could not develop intensively and did not become the ancestors of higher chordates.

Underwater "Venus flytrap"

(UROCHORDATA, or TUNICATA)

Tunicates are widely distributed in the oceans and seas. There are about 1100 species of them, of which about 1000 belong to the class of ascidians, leading an attached lifestyle. Most ascidians are solitary animals, the rest form colonies.

The body is covered with a thick shell - a tunic (which explains one of the names of the subtype), which forms a bag that communicates with the external environment with two wide tubes (siphons). Water enters the body through one of them, and it exits through the other (Fig. 68). The usual body size is a few centimeters.

The nervous system is poorly developed. It is represented by a small ganglion lying above the pharynx, and nerves extending from it to various organs. There is a thin skin-muscular sac.

Digestive system begins with a mouth that communicates with the external environment through the inlet siphon, and consists of the pharynx (there is an endostyle on its dorsal side), the stomach, and the horseshoe-shaped intestine, which opens with an anus into the outlet siphon. The pharynx is pierced by small gill openings that open into the peribranchial cavity. Reception of food (small organisms and organic pieces) and its digestion occurs, as in lancelets.

Rice. 68. Ascidia:

/- appearance, //- internal structure; 7 - oral siphon; 2- cloacal siphon; 3 - tunic (sheath); 4, 5 - mantle; 6 - pharynx; 7 - pharyngeal cavity; 8 - gill openings; 9 - endostyle; 10, 11 - peribranchial cavity; 12 - its wall; 13 - stomach; 14 - hepatic outgrowth; 15 - anus; 16 - testis; 17 - ovary; 18 - ducts of the sex glands; 19 - pericardium; 20 - heart; 21 - ganglion

The circulatory system is not closed. Blood is set in motion by the heart, from which vessels depart to various organs, especially strongly branched in the walls of the gill slits of the pharynx. The latter is very large and, like in lancelets, plays the role of a respiratory organ through which water passes, which is removed after gas exchange through the outlet siphon.

Dissimilation products are accumulated by some cells and remain in the body.

All tunicates are hermaphrodites; fertilization external and internal. Many species also reproduce asexually (by budding).

The position of tunicates in the animal system remained unclear for a long time, until A. O. Kovalevsky studied in detail the development of ascidians, showing that it is very similar to the development of lancelets and ends with the formation of a planktonic larva, similar in body shape to tadpoles and moving with the help of a tail. The larvae have a well-developed neural tube and notochord. After short period planktonic life, the larvae are attached to a solid substrate and their organization undergoes a radical restructuring, mostly regressive: the tail, together with the neural tube (with the exception of its anterior end, which turns into a ganglion) and the chord, are reduced (as superfluous during a sedentary lifestyle), while other organs, necessary for adult animals develop. The tunicates, thanks to the well-developed filtration apparatus, have become large group, which obtains food for itself in any place of the oceans and seas. The subtype is divided into 3 classes: ascidians, salps and appendiculars.

Ascidian class (ASCIDIAE)

The class includes about 1 thousand species. They are solitary or colonial sessile animals.

The body resembles a two-necked jar, attached by the base to the substrate and having two openings - oral and cloacal siphons. Outside, the body is covered with a tunic containing a fiber-like substance (the only case in the animal world of the formation of a substance close to cellulose). The tunic is secreted by the epithelium. Beneath it lies a skin-muscular sac, or mantle. The contraction or relaxation of the mantle muscles, together with the beating of the cilia of the epithelium of the inner walls of the oral siphon, contributes to the injection of water into the pharynx.

From the oral siphon, water enters the pharynx, which occupies most body. The walls of the pharynx are permeated with many holes - stigmas that open into the atrial cavity. The pharynx is followed by the esophagus, stomach, and intestine, which opens through the anus into the atrial cavity near the cloacal siphon. Passive power supply (filtering).

The pharynx also serves as a respiratory organ. The heart contracts now in one direction, then in the other direction with the same vessel. Like all tunicates, ascidians are hermaphrodites, but the sex glands do not develop simultaneously, and the same organism functions either as a male or as a female. Fertilization is external, rarely in the cloacal cavity.

The development of a fertilized egg leads to the formation of a tailed larva.

Salpa class (SALPAE, or THALIACEA)

The class includes 25 species living in warm seas. Representatives outwardly resemble a barrel, in which the oral and cloacal siphons are located at opposite ends of the body (Fig. 69). They swim due to jet propulsion: water is pushed out of the cloacal opening with force, due to which the animal moves in jerks. There is an alternation of asexual and sexual generations (metagenesis). Fertilized eggs produce asexual salps that reproduce by budding. Budding individuals form gonads and reproduce sexually. There is no dispersal larva characteristic of ascidians.

Zoologists of ancient times attributed the larval-chordates, or tunicates (Tunicata), to the type of molluscs. But already Lamarck in 1816 came to the conclusion that it would be more correct to consider these peculiar animals as an independent group of invertebrates, only remotely similar to mollusks. The famous works of A. O. Kovalevsky, devoted to the study of the history of the development of tunicates and lancelets, found out the well-known proximity of larval-chordates to non-cranial and vertebrates. This proximity is indicated by: the picture of the development of the embryonic layers of the tunicates, the respiration associated with the anterior intestine, the formation of the rudimentary chord and its position relative to the intestine and neural tube.

Following short definition can characterize tunicates. These are chordate animals, in which the notochord is located exclusively in the caudal region of the body, it usually exists in the larval period of development and disappears at the end of this period. The single-layer epithelium of the skin secretes a gelatinous membrane (tunic) that covers the entire body of the animal. The pharynx looks like a gill box. Reproduction occurs partly sexually, partly by budding; there is a change of generations. Almost all species are hermaphroditic. Currently, there are up to 1500 species of tunicates, of which the vast majority lives on the bottom; some floats in the water column and is part of the plankton. The size of animals belonging to this subtype ranges from 1/2 millimeter to 400 millimeters, rarely more. Colonial forms sometimes form ribbons several meters long. The subtype contains 3 classes: sea ​​squirts(Ascidiae) salups(Salpae) appendiculars(Appendiculariae).

Fig.1. hullers

Top row - ascidians, from left to right: ascidia mentula, colony of botrillus Schlosser, clavelin, gastric cyonia. Bottom row, from left to right: appendicularia oikopleura, cask goblet dolioletta, colony of salp piebald, Atlantic pyrosome

A group of primitive chordates, which in the larval stage of development have all the characteristic Chordata type structural features, but upon transition to the adult state, they lose the chord and experience a deep transformation of the central nervous system, which turns from the neural tube into a compact nerve ganglion (only the appendiculars retain the chord and neural tube all their lives!. The simplification of the body with the age of animals is associated with the transition from the mobile existence of larvae to immobile adults.

Specific features of the structure: there is a skin-muscular sac (epithelium and layers of longitudinal and annular muscles); circulatory system open, tubular heart, pendulum circulation; the nervous system is represented by a nerve ganglion that does not have an internal cavity, from which nerve cords depart; there is no excretory system; hermaphrodite, fertilization external environment. Ascidia and salps also reproduce asexually.

Fig.2. Similarities and differences between larval-chordates and non-cranial

The body of tunicates is never segmented, although in some ascidians it has a noticeable division into 2 or 3 sections. Outside, the body is dressed in a gelatinous, leathery or cartilaginous sheath-tunic. It is based on a substance that is extremely close to plant fiber (cellulose).

Musculature. Beneath the outer epithelium lies a layer connective tissue with the muscles enclosed in it; the musculature of ascidians consists of longitudinal and transverse muscle fibers; in salps it forms a series of rings.

Nervous system. The central nervous system in adult tunicates consists of a single node on the dorsal surface with nerves extending from it.

The sense organs are poorly developed: there is an eye in the form of a pigment spot on the nerve ganglion, sometimes with a refracting body (in ascidian larvae, in salps, pyrosis), an auditory organ in the form of an unpaired otocyst (in ascidian larvae, in Doliolum), organs of touch in the form outgrowths on the edges of the inlet and outlet openings. Beneath the ganglion, the wall of the gill sac protrudes, forming an organ that has been compared to the Hypophysis of the vertebrate brain.

Digestive system. Most characteristic feature the intestinal canal is a strong development of the anterior section, which serves as an organ of respiration and eating. In the appendiculars, the wall of this section (gill sac) is pierced by only two openings that open directly to the outside; in ascidians, the wall of the gill sac is equipped with numerous openings (gill slits) that open into the so-called peribranchial or perithoracic cavity, which surrounds most of the wall of the gill sac and constitutes the anterior part of the cloacal cavity. Blood s-ma. The heart lies on the ventral side of the body; the appendiculars have no blood vessels; the rest of the tunicates have an anterior and posterior vessel departing from the heart. A remarkable feature of O.'s blood circulation is that the heart contracts for some time in a certain direction, then the contractions stop and then start again, but in the opposite direction; Therefore, the movement of blood does not have a definite direction, and in each vessel and in the heart the blood moves first in one direction, then in the other.

Reproductive system and features of reproduction. 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 at different times, and therefore self-fertilization is impossible. In ascidians, salps, and pyrosomes, the gonadal ducts open into the cloacal cavity, and in appendicularia, spermatozoa enter the water through ducts that open on the dorsal side of the body, while eggs can only come out after the body walls rupture, 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, fertilization of eggs occurs in the cloacal cavity of the mother, where spermatozoa of other individuals penetrate with a stream of water through siphons, and fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then go outside, i.e. there is a kind of rebirth.

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 reproductive 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 ripeness of the pharynx, perceiving the corresponding signal from the water. Through the nervous system, it accelerates the maturation of the gonads.

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, whose body 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 the 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. Throat pierced a large number 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 a typical adult baggy appearance. A tunic quickly forms on the surface of the body. The hullers 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, difficult gill apparatus, endostyle, reproduction not only sexually, but also by budding.

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

In spite of big variety organisms, all of which have a number common features structure and development.

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, at 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. Lower type organisms have gills on them.

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 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 warm time of the year. 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 subtype Vertebrates appeared only in the Ordovician-Silurian, but in jurassic all currently known types and classes already existed.

Total modern species a little over 40,000

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 germinal 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 they are species diversity not great.

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