The circulatory system of ascidians. Class Ascidia - Ascidiacea

Image of sea squirts from the atlas of Ernst Haeckel Kunstformen der Natur 1904
Top to bottom, left to right:

Cynthia melocactus
Synoecum turgens
Botryllus schlosseri = Botryllus polycyclus
Botryllus marionis
Polyclinum constellatum
Polyclinum constellatum
Cynthia melocactus
Cynthia melocactus
Polycyclus cyaneus
Sidnyum elegans, = Fragarium elegans
Molgula tubulosa
Botryllus rubigo
Botryllus helleborus
Botrylloides purpureus

1.4. Digestive and nutritional system

The opening of the oral siphon leads into the mouth, surrounded by tentacles. Next is a large saccular pharynx, the walls of which are pierced by numerous gill openings - stigmas, which open into the atrial, or navcolostrum, cavity. An endostyle runs along the ventral side of the pharynx - a groove lined with blinking epithelium has glandular fields, the mucus secreted by them contains thyroid-stimulating hormone. On the opposite side, a thin movable fold - the dorsal plate - enters the pharyngeal cavity. Movement of cilia ciliated epithelium, bordering the edges of the stigmas, create a flow of mucus secreting the endostyle in the direction of the dorsal plate. This creates a continuous veil of mucus that traps food particles. due to the back of the planus, a mucous tourniquet is formed, which slides into the esophagus at the bottom of the pharynx, which expands into a sac-like stomach. The latter passes into the intestine itself, which opens with the anus into the naphthalmic cavity. Small particles enter the throat along with water. marine organisms. Undigested food debris from the atrial cavity is expelled through the cloacal siphon.

1.5. Extraction system

Most excretory organs ascidian are numerous vesicles hanging into the atrial cavity of the storage bud; in some species one large vesicle develops. They are located along the walls of the mantle, filled with uric acid crystals, and the blisters are not removed throughout the life of the individual. In some colonial sea squirts (Botryllus), the products of nitrogen metabolism are excreted into environment in the form of ammonia, at the same time uric acid nodules accumulate in the accumulation kidneys.

1.6. Nervous system

Nervous system consists of a ganglion, which is located between the oral and cloacal siphons, and does not have an internal cavity - the neurocoel. From the ganglion along the dorsal side of the body, nerves extend to the oral opening, as well as to the genitals.

1.7. Sense organs

There are no sensory organs, with the exception of the tentacles, which perform the function of touch.

2. Reproduction and development

All tunicates are hermaphrodites. Reproduction occurs by both sex and budding. During asexual reproduction, a protrusion (stolon) is formed on the mother's ventral side, including internal organs. Kidneys are formed on the stolons, in which all future organs are formed. Then the single buds are separated, but the colonial ones remain and continue to reproduce by budding. Sexual reproduction studied by A. Kovalevsky. The gonads of ascidians mature at different times; the same individual functions either as a male or as a female. Ripe eggs are released through the genital ducts into the nasal cavity, where fertilization occurs with sperm that penetrate with water. Fertilized eggs are carried out through the cloacal siphon.

Halocynthia papillosa off the coast of Croatia

2.1. Larva

From the fertilized egg, a fork-swimming larva is formed, which is not similar to the adult forms. The larvae have fewer gill openings. On the dorsal side of the embryo, the nervous system is formed from the ectoderm, which appears in the form of a plate, the edges of which are wrapped, forming a groove, and closes. Then into the neural tube, the cavity of which is a typical neurocoel. In the neural tube lies the notochord, formed from the dorsal part of the endodermal rudiment of the intestine, and on the sides of it, mesodermal stripes give rise, among other things, to the muscles. The mouth develops as an ectodermal cavity that grows towards the endodermal gut. From the wall of the latter, gill protrusions develop, which break through openings into the ectodermal ovary (atrial) cavity. As a result, a fork-swimming larva with all the main characteristics of chordates is formed. It stays in this state for several hours, or for one day. With the help of suckers, which are located at the anterior end of the body, it attaches to an underwater object and turns into a sessile ascidian, and the restructuring of its organization is in the nature of a regressive metamorphosis. The tunic begins to develop quickly. The notochord disappears, decreases in size, and then the neural tube, sensitive vesicle, and point disappear. However, the back part of the vesicle remains from the nervous system, which forms a ganglion (nerve ganglion). Further, the pharynx itself expands, the number of gill slits increases. The oral and anal siphons move upward. The body takes on a sac-like shape, giving off a tunic. That is, besides the numerous gill openings opening into the atrial cavity, almost no signs are preserved that would indicate that these animals belong to chordates. So, regressive changes are expressed very clearly in ascidians, and, obviously, this regression is associated with the transition to a sedentary lifestyle, in which both the muscles and sensory organs with a highly developed nervous system have lost their importance. The vitilion-swimming ascidian larva has all the characteristics of chordates: notochord, spinal cord, complex sensory organs in the brain vesicles. Attaching to the bottom, it changes and loses these characteristics.

It leads a sedentary lifestyle and is attached with the help of its sole to underwater rocks and ledges, sometimes found in very significant numbers. The larvae do not resemble adult animals and swim freely in plankton. The body of the ascidian has the appearance of an oblong irregular shape a bag, which with its base - the sole - is attached to the substrate (to underwater objects), and with its free end it extends under water. If you touch the body of an ascidian, the irritated animal shrinks somewhat and is forcefully thrown out of two holes located at the free end of the animal.

In this case, you can see that at the free end of the body one opening, the oral one (oral siphon), is located slightly higher than the other, which is called the atrial or cloacal (cloacal siphon; Fig. 1, 2). If the ascidian is removed from the water, both openings sharply contract and almost close due to the contraction of the sphincter muscles. When the animal is in a calm, normal state, a current of water circulates through its body, entering the mouth opening and exiting through the atrial opening.

Body walls. The body is dressed in a single layer prismatic epithelium, on its surface a thick tunic shell (Fig. 1.10). Epithelial cells contain pigment granules that give the animal a certain color. As noted in general characteristics, the shell has a different consistency; it contains the substance tunicin, which is extremely close to plant fiber, or cellulose. Under the Tunica lies an ectodermic single-layer epithelium, and under it is a connective tissue layer with muscle fibers. Muscles consist of a layer of external (longitudinal) and internal (circular) fibers. The epithelium, together with the connective tissue layer and muscles, forms the body wall, or mantle. It envelops the body under the tunic, and at the locations of the two openings mentioned above is elongated into short and wide tubular outgrowths called siphons: oral and cloacal, or atrial. When opening the tunic, it is clear that the body lies freely in the latter and fuses with the tunic in two places: around the oral and cloacal openings (Fig. 1, 14, 15).

Rice. The structure of ascidians.

I-whole ascidian: (side); II - ascidian in longitudinal section;III-part of the pharyngeal wall (with vessels and stigmata);IV-longitudinal rashrea of ​​the anterior part of the body in the area of ​​the oral opening and the nerve ganglion.

1-mouth opening in the area of ​​the upper siphon; 2-cloacal opening in the area of ​​the lower siphon; 3-tentacles in front of the entrance to the pharynx; 4-throat; 5-transverse vessel; 6-stigma; 7-endo-style; 8-dorsal longitudinal outgrowth (lamina dorsalis); 9-heart; 10-tunic; I-stomach; 12 - testis; 13-ovary; 14-anal opening; 15-hole into the esophagus (from the pharynx); 16-nervous node; 17-dorsal nerve trunk; 18-paranervous gland; 19-epithelium; 20 - sole of ascidians.

Atrial cavity. Between the walls of the body there is a cavity lined with ectoderm. This cavity surrounds the wide pharynx, which only on the ventral side fuses with the mantle, and therefore in this part is not covered by the atrial cavity. The described cavity opens outwards with the atrial or cloacal opening. This cavity cannot be considered the cavity of the Body (the whole); the atrial cavity probably developed from the protrusion of the outer surface of the body inward.

Ascidian pharynx and intestines

The oral opening leads through a wide and short passage into the voluminous pharynx (pharynx), also sometimes called the gill cavity (Fig. 1, 4). Before the entrance to the pharynx there is a zone of horizontally located thin irreducible tentacles, the number of which is 6, and sometimes more (Fig. 1, 3). The delicate and thin walls of the pharynx are pierced by many gill slits, or stigmas, located in several horizontal and vertical rows. It is clear that the gill slits are not visible when examining the ascidians from the outside, since these slits open into the atrial (peribranchial) cavity, located, as indicated, between the walls of the body. The edges of each stigma are equipped with long hairs, which oscillatory movements they drive a flow of water sequentially from the oral opening to the pharynx, into the atrial cavity and up to the atrial, or cloacal, opening.

Endostyleand dorsal longitudinalascidian growth

It was mentioned above that the pharynx along the ventral side of the body fuses with the mantle. Here, a special subbranchial groove, or endostyle, extends along the ventral side of the pharynx (Fig. 1, 7). The large cells that make up the endostyle are of two kinds: glandular, secreting an adhesive mucous substance, and ciliated, bearing the finest hairs on their free surfaces. A current of water passes past the endostyle; organic (food) particles are enveloped in the sticky mucous secretion of the corresponding endostyle cells, and the thin cilia of other endostyle cells are driven further to the peripharyngeal ring, which actually consists of two rings, between which the peripharyngeal groove is located. On the dorsal side of the body, the peripharyngeal ring passes into the dorsal longitudinal process opposite to the endostyle (Fig. 1, 8) (lamina dorsal is). With the help of the cilia of the cells of this outgrowth, food parts (in the form of lumps surrounded by mucus) are transferred to the esophagus, to which the dorsal outgrowth approaches.

The intestine begins with the esophagus, the opening of which opens at the bottom of the dorsal wall of the pharynx at the lower edge of the dorsal process (Fig. 1, 15). The stomach is spindle-shaped with rather thick walls; the intestine, forming a double loop, opens into the cloacal cavity. On a cross-section of the intestinal tube, one can see that a longitudinal narrow fold, typhlosolis, hangs down from its upper wall, reminiscent in its location of the corresponding fold in many annelids. The liver is completely absent. It should be noted that the walls of the stomach have a glandular structure and that a system of thin tubes branches along the walls of the intestines. Apparently, these tubes play the role of digestive glands.

Circulatory system

Having the shape of an elongated sac, it is located on the abdominal side of the animal and is lined with a thin pericardial sac (Fig. 1, 9). Contractions of the heart occur in a very unique way: having a peristaltic nature, they are separated by some pause and occur from one end of the heart to the other. Thus, the direction of the blood flow sent from the heart changes all the time (at certain short intervals, or pauses). A large blood vessel runs from the two opposite ends of the heart. The large branchial artery (arteria branchio-cardiaca) originates from the anterior end of the heart; it stretches along the middle of the ventral side and sends out numerous branches to the rows of stigmas, as well as secondary small whorls between the stigmas. Thus, the entire gill region turns out to be widely supplied with a whole complex network of blood vessels. The visceral intestinal artery (arteria cardio-visceralis) departs from the posterior dorsal side of the heart, breaking up into branches going to the internal organs. In this area, blood vessels form lacunae, reminiscent in structure of formations known from bivalves. This entire system of blood vessels and lacunae opens up significantly ny gill-intestinal sinus (sinus branchio-visceralis), stretching along the middlelines of the dorsal part of the pharynx and connecting to the dorsal ends of the transverse gill vessels.

When it contracts from the dorsal to the abdominal region, it rushes along the branchial artery to the ventral side of the pharynx, then along the transverse branches. oxidizes between the stigmas and flows into the enterobranchial sinus; The further path of the blood flow passes through the lacunae and intestinal vessels to the posterior dorsal side of the heart. With the subsequent contraction of the heart, the blood flow has the opposite direction, that is, it initially rushes into the intestinal artery.

Nervous system ascidians

It consists of a single suprapharyngeal or cerebral nerve ganglion, which is located on the dorsal side of the body between the oral and cloacal openings (Fig. 1, 16). The nerves that go to the edges of the oral opening, as well as the nerve that goes to the posterior part of the body, depart from the nerve ganglion. Between the nerve ganglion and the pharynx there is a small gland, the narrow duct of which widens at the end and flows into the dorsal part of the pharynx (Fig. 1, 18). This paranervous gland is considered a homologue of the pituitary gland, i.e., the lower cerebral appendage of vertebrates. This homology, however, is highly problematic.

There are solitary and colonial ascidians(in the latter case, individual animals are more or less closely connected to each other). By appearance a single ascidian resembles a two-necked jar, tightly attached at the base to the substrate and having two openings - the oral and cloacal (atrial) siphons. The body is covered on the outside with a tunic that has a complex structure: it is covered with a thin, usually hard cuticle, under which lies a dense fibrous network containing a fiber-like substance - tunicin and acidic mucopolysaccharides.

Tunic secreted by the epithelium and usually impregnated with inorganic salts, turning into an elastic and dense protective shell. Individual epithelial and mesenchymal cells, and often blood vessels, penetrate into it. In some species of ascidians, the tunic is thin, smooth, translucent, sometimes gelatinous or jelly-like, while in others it is thick and lumpy. In the ascidian Ciona, the shell is formed of three layers of fibers; it contains about 60% tunicine, 27% protein and 13% inorganic substances. In some species, the tunic fits tightly to the ectoderm, while in others it fuses with it only at the edges of the siphons.

Under the tunic lies mantle or skin-muscular bag from a single-layer skin epithelium (ectoderm) and two or three layers of longitudinal and transverse muscle bundles fused with it, lying in loose connective tissue. In the area of ​​the siphons there are special annular bundles of muscles that close and open these openings. Contraction and relaxation of the pallial muscles, along with the flickering of the cilia of the epithelium of the inner walls of the oral siphon, ensures the pumping of water into the pharynx.

Oral siphon leads into a huge pharynx, which occupies most of the ascidian’s body. The boundary between the inner surface of the oral siphon and the walls of the pharynx is formed by a thickened annular ridge - the peribranchial or peripharyngeal groove, along which thin tentacles invisible from the outside are located; in some species there are up to 30 of them. The walls of the pharynx are pierced by many small gill openings - stigmata, which open not outward, but into the atrial cavity. A short esophagus extends from the bottom of the pharynx, turning into an extension - the stomach, followed by the intestine, which opens with the anus into the atrial cavity near the cloacal siphon. Along the ventral side of the pharynx runs an endostyle - a groove lined with ciliated epithelium and having glandular fields; the mucus they secrete contains thyroid hormones. On the opposite side, a thin movable fold protrudes into the pharyngeal cavity - the dorsal groove, or plate. The movements of the cilia of the ciliated epithelium bordering the edges of the gill openings (stigmas) create a flow of mucus secreted by the endostyle near the inner walls of the pharynx towards the dorsal plate. This is how a continuously moving veil (“network”) of mucus arises, trapping food particles from water entering the pharynx through the oral siphon, flowing through the gill openings into the atrial cavity and out through the cloacal siphon. Streams of mucus with trapped food particles at the dorsal plate turn into a mucous rope that flows into the esophagus. In the stomach and intestines, food is digested and absorbed, and undigested residues are thrown out through the anus into the atrial cavity and are discharged out with a stream of water. On the stomach walls of some species there are folded or tuberculate protrusions called hepatic outgrowths. They, however, cannot be considered an analogue of the liver of higher chordates. The tubular pyloric glands, which secrete digestive enzymes, are located in the wall of the stomach.

The pharynx also serves as a respiratory organ. The circulatory system of tunicates is unique. The heart has the appearance of a short tube, from one end of which a vessel runs along the dorsal plate, branching in the walls of the pharynx; vessels extending from the other end of the heart are directed to the internal organs (stomach, intestines, gonads, etc.) and the mantle, where they pour blood into small cavities - lacunae. The heart sequentially, over the course of several minutes, contracts first in one direction, then in the opposite direction. Therefore, the blood is directed either to the internal organs and mantle, or to the walls of the pharynx, where it is saturated with oxygen. Thus, blood circulation is replaced by a pendulum-like movement of blood through the same vessels, alternately performing the function of arteries and veins. This type of "circulation" appears to reduce friction of the viscous fluid (blood) in the very complex network of vessels of the colossal pharynx, while at the same time providing a relatively low oxygen requirement for these sessile animals.

In the blood of ascidians there are cells - vanadocytes containing vanadium and free sulfuric acid, the concentration of which reaches 9%; they make up 98% of blood cells. There are also cells containing green bodies, consisting of iron combined with protein. The blood and tissues of ascidians contain relatively large amounts of Ti, Cr, Si, Na, Al, Ca, Fe, Mn, Cu, and Ni. All this emphasizes the high biochemical specificity of ascidians (and tunicates in general).

The pharynx and most of the internal organs are surrounded by an atrial cavity, which opens to the outside with a cloacal siphon; the walls of the atrial cavity are lined with ectoderm. Mesenteric adhesions develop between the body wall - the mantle - and the walls of the pharynx. The formation of the atrial cavity increases the flow of water through the pharynx, intensifying both breathing and food acquisition. On the wall of the mantle facing the atrial cavity, sometimes on the intestinal walls there are small swellings - renal vesicles (in some species one large vesicle develops). In such “storage buds” uric acid crystals accumulate, the removal of which from the vesicles does not occur during the life of the individual. Some colonial sea squirts ( Botryllus) products of nitrogen metabolism are excreted from the body into the environment in the form of ammonia (a property of many invertebrates); at the same time, uric acid nodules accumulate in the “renal vesicles”.

Ascidians, like other tunicates, are hermaphrodites. Typically, paired ovaries in the form of long, egg-filled sacs lie in the coelom cavity and are attached to the walls of the mantle; short tubular oviducts open into the atrial cavity near the cloacal siphon. Some species have up to a dozen small, rounded ovaries. The testes, in the form of numerous lobules or compact oval bodies, are also located on the walls of the mantle; their short ducts open into the atrial cavity. Self-fertilization is prevented by the fact that in each individual the sex cells mature at the same time, and therefore it functions either as a male or as a female. Fertilization of eggs occurs in water outside the body or in the cloacal siphon, where sperm penetrate with a current of water through the gill openings. Fertilized eggs are removed from the cloacal siphon and develop outside the body. However, in some ascidians, the development of eggs occurs in the cloacal cavity, and the formed larvae, after the rupture of the egg membranes, float out.

For successful reproduction of sessile animals, synchronization of maturation of germ cells in neighboring individuals is especially important. It is provided by a special mechanism. The reproductive products (eggs and spermatozoa) released from the first mature individuals pass through a stream of water to neighboring animals. In this case, they are partly captured by the ciliated funnel of the subneural gland, associated with the peribranchial groove and closely adjacent to the nerve ganglion located on the dorsal side of the animal. The reproductive products activate the secretion of the subneural gland, and the latter excites the nerve ganglion, which in turn activates the activity of the gonads through the nerves going to them. Such neurohumoral regulation in a short time involves animals in the reproduction of a large territory.

As a result of the development of a fertilized egg, a tailed larva is formed, which differs sharply in structure from adult ascidians. It has a small oval body and is quite long tail. A small oral opening leads into the pharynx, which is not yet penetrated by gill openings, but already has a formed endostyle. Behind the pharynx there is an intestine that ends blindly, in which differentiation into sections is planned. As a result of separation from the ectoderm, a neural tube appears, the anterior end of which forms an extension - the medullary vesicle; in the latter, the pigment eye and statocyst are formed. The brain vesicle opens with a hole in the initial part of the pharynx (during metamorphosis, a ciliated fossa will form in place of this hole). Behind the pharynx, the notochord begins - an elastic cord of highly vacuolated cells, continuing almost to the end of the tail; the neural tube is located above the notochord. On the sides of the notochord lie muscle cells, the number of which varies little among different species. At this stage, the larva, several millimeters long, ruptures the egg shells and, emerging into the water, swims, working its tail like a frog tadpole. On the dorsal part of the body behind the brain vesicle, paired depressions form, then merge together and grow into the pharynx; This is how the atrial cavity appears. At the same time, gill openings break through the walls of the pharynx; in larvae of different species their number varies from 2 to 6, rarely more. At this stage, the ascidian larva has the main characteristic features of chordates (notochord, neural tube located above it, pharynx with gill openings), but it does not feed.

MATERIAL AND EQUIPMENT

SYSTEMATIC POSITION OF THE OBJECT

Topic 1. STRUCTURE OF TUNATS

Phylum Chordata, Chordata

Subtype Tunicates, Tunicata

Ascidia class, Ascidiae

Representative – Ascidia, Ascidiae sp.

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Tables: The structure of the ascidian, the diagram of the structure of the ascidian larva, the successive stages of metamorphosis of the ascidian larva, the structure of the salp and barrel, the structure of the appendicular.

For one or two students you need:

1. Fixed ascidian.

2. Fixed salps placed in Petri dishes in water (black paper must be placed under the Petri dish.

3. Wet preparations of ascidians and salps.

4. Bath.

5. Preparation needles – 2.

6. Hand magnifier 4-6 X.

EXERCISE

Consider the appearance of a fixed solitary ascidian, a solitary salp and a colonial ascidian - pyrosoma. Make the following drawings:

1. Scheme internal structure ascidians.

2. Scheme of the structure of an ascidian larva.

3. Scheme of the structure of a single salp.

4. Scheme of the structure of a pyrosome.

In appearance, a single ascidian resembles a two-necked jar (Fig. 1), tightly attached at the base to the substrate and having two openings - oral and cloacal (atrial) siphons. The body is covered on the outside with a tunic that has a complex structure: it is covered with a thin, usually hard cuticle, under which lies a dense fibrous network containing a fiber-like substance - tunicin (this is the only case of the formation of large quantity substances close to plant fiber (cellulose) and acidic mucopolysaccharides. The tunic is secreted by the epithelium and is usually impregnated with inorganic salts, turning into an elastic and dense protective shell. Individual epithelial and mesenchymal cells, and often blood vessels, penetrate into it. In some species of ascidians, the tunic is thin, smooth, translucent, sometimes gelatinous or jelly-like, while in others it is thick and lumpy. In some species, the tunic fits tightly to the ectoderm, while in others it fuses with it only at the edges of the siphons.

Under the tunic lies a mantle or skin-muscular sac made of a single-layer skin epithelium (ectoderm) and two or three layers of longitudinal and transverse muscle bundles fused with it, lying in loose connective tissue. In the area of ​​the siphons there are special annular bundles of muscles that close and open these openings. Contraction and relaxation of the pallial muscles, along with the flickering of the cilia of the epithelium of the inner walls of the oral siphon, ensures the pumping of water into the pharynx.

The oral siphon leads into the huge pharynx (Fig. 1, 4 ), occupying most of the body of the ascidian. The boundary between the inner surface of the oral siphon and the walls of the pharynx is formed by a thickened annular ridge - the peribranchial or peripharyngeal groove, along which thin tentacles invisible from the outside are located; in some species there are up to 30 of them. The walls of the pharynx are penetrated by many small gill openings - stigmas, which open not outward, but into the atrial cavity. A short esophagus extends from the bottom of the pharynx, passing into an extension - the stomach, followed by the intestine, which opens with the anus into the atrial cavity, near the cloacal siphon (Fig. 1, 14 ). An endostyle runs along the ventral side of the pharynx (Fig. 1, 7 ) - a groove lined with ciliated epithelium and having glandular fields; the mucus they secrete contains thyroid hormones. On the opposite side, a thin movable fold protrudes into the pharyngeal cavity - the dorsal groove, or plate (Fig. 1, 8 ). The movements of the cilia of the ciliated epithelium bordering the edges of the gill openings (stigmas) create a flow of mucus secreted by the endostyle near the inner walls of the pharynx towards the dorsal plate. This is how a continuously moving veil (“network”) of mucus arises, trapping food particles from water entering the pharynx through the oral siphon, flowing through the gill openings into the atrial cavity and through the cloacal siphon to the outside. Streams of mucus with trapped food particles at the dorsal plate turn into a mucous rope that flows into the esophagus. In the stomach and intestines, food is digested and absorbed, and undigested residues are thrown out through the anus into the atrial cavity and are discharged out with a stream of water. On the stomach walls of some species there are folded or tuberculate protrusions called hepatic outgrowths. They, however, cannot be considered an analogue of the liver of higher chordates. The tubular pyloric glands, which secrete digestive enzymes, are located in the wall of the stomach.

The pharynx also serves as a respiratory organ. The circulatory system of tunicates is unique. Heart (Fig. 1, 9 ) has the appearance of a short tube, from one end of which a vessel runs along the dorsal plate, branching in the walls of the pharynx; vessels extending from the other end of the heart are directed to the internal organs (stomach, intestines, gonads, etc.) and the mantle, where they pour blood into small cavities - lacunae. The heart sequentially, over the course of several minutes, contracts first in one direction, then in the opposite direction. Therefore, the blood is directed either to the internal organs and mantle, or to the walls of the pharynx, where it is saturated with oxygen. Thus, blood circulation is replaced by a pendulum-like movement of blood through the same vessels, alternately performing the function of arteries and veins. This type of "circulation" appears to reduce the friction of the viscous fluid (blood) in the very complex network of vessels of the colossal pharynx, while at the same time providing a relatively low oxygen requirement for these sessile animals.

The pharynx and most of the internal organs are surrounded by an atrial cavity, which opens to the outside with a cloacal siphon (Fig. 1, 2 ); the walls of the atrial cavity are lined with ectoderm. Mesenteric adhesions develop between the body wall - the mantle - and the walls of the pharynx. The formation of the atrial cavity increases the flow of water through the pharynx, intensifying both breathing and food acquisition. On the wall of the mantle facing the atrial cavity, sometimes on the intestinal walls there are small swellings - renal vesicles (in some species one large vesicle develops). In such “storage buds” uric acid crystals accumulate, the removal of which from the vesicles does not occur during the life of the individual. In some colonial ascidians, the products of nitrogen metabolism are excreted from the body into the environment in the form of ammonia (a property of many invertebrates); at the same time, uric acid nodules accumulate in the “kidney vesicles.”

Ascidians, like other tunicates, are hermaphrodites. Usually paired ovaries (Fig. 1, 13 ) in the form of long, egg-filled sacs lie in the coelom cavity and are attached to the walls of the mantle; short tubular oviducts open into the atrial cavity near the cloacal siphon. Some species have up to a dozen small, rounded ovaries. Testes (Fig. 1, 12 ) in the form of numerous lobules or compact oval bodies are also located on the walls of the mantle; their short ducts open into the atrial cavity. Self-fertilization is prevented by the fact that in each individual the sex cells do not mature at the same time, and therefore it functions either as a male or as a female. Fertilization of eggs occurs in water outside the body or in the cloacal siphon, where sperm penetrate with a current of water through the gill openings. Fertilized eggs are removed from the cloacal siphon and develop outside the body.

As a result of the development of a fertilized egg, a tailed larva is formed, which differs sharply in structure from adult ascidians (Fig. 2). The ascidian larva has the main characteristic features of chordates: a notochord (Fig. 2, 11 ), the neural tube located above it (Fig. 2, 9 ), pharynx with gill openings (Fig. 2, 19 ), but she doesn't eat.

The free-swimming larval stage lasts only a few hours. At the anterior end of its body, ectodermal growths are formed.

you are the papillae of attachment (Fig. 2, 1 ), secrete sticky mucus. With their help, the larva, having found suitable soil, attaches to an underwater object (stone, large shell, etc.) and undergoes a regressive metamorphosis. The tail (notochord, neural tube, muscle cells) undergoes resorption and gradually disappears. The pharynx grows, in which the number of gill openings sharply increases; the intestinal tube differentiates, and its end breaks into the enlarged atrial cavity. At the same time, it is formed circulatory system, gonads (sex glands) are formed, the oral and cloacal siphons move, and the body acquires a sac-like appearance characteristic of an adult ascidian. During metamorphosis, the pigment eye disappears (Fig. 2, 5 ) and statocysts (Fig. 2, 4 ), A nerve cells The walls of the medullary vesicle are grouped into a compact nerve ganglion - the dorsal ganglion.

In addition to sexual reproduction, asexual reproduction is also widespread in ascidians. The ascidian, which has developed from a fertilized egg, settled to the bottom and undergone metamorphosis, grows; then, in the lower part of its body, an outgrowth is formed - a kidney stolon (sometimes there are several of them), into which the processes of all internal organs grow. At the end of the stolon, swellings are formed - buds; in each of them, through complex differentiation, the organs of an adult sexual individual are formed. The animals formed as a result of budding either break away from the stolon, fall to the ground and attach next to the maternal organism (single ascidians), or maintain a close connection with it (colonial ascidians).

A specialized and short-lived phoretic larva, developing from a fertilized egg, gives ascidians the opportunity, when settling, to occupy parts of the seabed distant from the place of birth.

The ascidian class unites three orders: solitary ascidians ( Monascidiae), compound ascidians ( Synascidiae) and fireflies ( Pirosomata).

Pyrosomes, salp-like colonial ascidians, occupy a separate position (Fig. 3). A colony is formed by budding. From the fertilized egg, an ascidian-like zoocide develops - the founder of the colony. By budding, a group of four cross-shaped individuals appears, lying in a common tunic. On their abdominal stolons, buds are formed, which, transforming into zooids, tear off from the stolon and occupy a certain position in the tunic. As a result, a colony appears in the form of a cone or a tube closed at one end (Fig. 3, A); it can include several hundred individual individuals - zooids (Fig. 3, B, 15 ). Their oral siphons open on the surface of the colony (Fig. 3, 2 ), and cloacal - into its internal cavity (Fig. 3, 9 ). Thanks to this arrangement of siphons, the colony is capable of jet propulsion. A finger-like outgrowth of the tunica is formed near the oral siphon of each zooid (Fig. 3, 1 ). There is no mobile dispersal larva. These animals are called fireflies because on the sides of the front part of the pharynx of each zooid there are groups of cells in which symbiotic luminous bacteria live (Fig. 3, 6 ).

Salps are swimming (pelagic) marine animals that have structural features in common with ascidians, but differ in the ability for reactive movement - the oral and cloacal siphons are located at opposite ends of the body (Fig. 4), surrounded by a thin, gelatinous, translucent tunic (Fig. 4, 1 ). The mantle is formed by a single-layer epithelium, to the inner surface of which muscle bands are adjacent, like hoops covering the body of the animal (Fig. 4, 6 ). Unlike adult ascidians, which have smooth muscles, salpas have striated muscle fibers. Almost the entire body is occupied by the pharyngeal and atrial cavities, separated by a septum - the dorsal process. This septum is pierced by several gill openings - stigmas. A well-developed endostyle runs along the bottom of the pharynx (Fig. 4, 3 ). A short esophagus extends from the back of the pharynx, passing into the stomach (Fig. 4, 10 ); the intestine opens into the atrial cavity through the anus (Fig. 4, 11 ). The heart lies under the esophagus (Fig. 4, 12 ). In the front part of the body on the dorsal side there is a nerve ganglion (Fig. 4, 5 ), to which the pigmented ocellus (the organ of light perception) presses. Under the ganglion there is a neural gland, and at some distance from it lies the organ of balance - the statocyst, connected to the ganglion by a nerve (Fig. 4, 4 ).

Salps are characterized by alternation of sexual and asexual generations (metagenesis), usually associated with the formation of complex polymorphic colonies.

Representatives of the third class of tunicates - appendicularia - are similar in appearance and structure to ascidian larvae. These are small floating tunicates ranging in length from a few millimeters to 1–2 cm, lacking a true tunica and an atrial cavity; There is only one pair of gill openings in the pharynx. The notochord is surrounded by a thin connective tissue membrane. Above the notochord lies the nerve trunk, and on its sides stretch two muscle cords formed by giant cells. The body is surrounded by a transparent house, the shape of which varies among different species. They reproduce only sexually. Thus, appendicularia have no alternation of generations, no asexual reproduction, and no distinct larval stage.

Before moving on to the story about sessile marine animals such as chordates, a few introductory words.

The animal world is traditionally divided into vertebrates and invertebrates. Even zoologists are trained at universities in two different departments: vertebrate zoology and invertebrate zoology. But we must be aware that vertebrates and invertebrates are not at all equivalent groups. If invertebrates include living creatures of several types, then vertebrates are only part of the chordate type.

Chordates are not the largest type of animal kingdom, but its representatives have mastered all habitats. This type includes three groups (subtypes) of organisms. Tunicates or Larval Chordates ( Tunicata , or Urochordata ) traditionally belong to invertebrate animals.
Craniumless or Cephalochordates ( Acrania or Cephalochordata ) - combining several types - lancelets, as a rule, are studied in the introductory chapters of a course in vertebrate zoology.
And to the actual vertebrates or cranial ones ( Vertebrata or Craniata ) animals include representatives of classes cartilaginous fish, bony fish, amphibians, reptiles, birds and mammals.

All invertebrates of the tunicate subphylum are marine animals. They got their name due to the fact that their body is covered on the outside with a gelatinous shell-tunic. This subtype is usually divided into 3 classes: Appendicularia (Appendiculars), Thaliacea (Pelagic tunicates or salps) And Ascidiacea (Ascidians). Representatives of the first two classes swim freely in the water column, but adult ascidians lead an attached lifestyle, but they also have a free-swimming tailed larva.

The class of ascidians includes more than 2.5 thousand species inhabiting the seas and oceans of all latitudes. Ascidians are solitary and colonial. They lead a stationary lifestyle, attaching their base to the bottom, piers or bottoms of ships. A typical adult solitary ascidian most closely resembles a sac with two siphons attached to the bottom, pumping water through itself. Through one of the siphons, water enters the body, and, having given it oxygen and edible particles, exits through another siphon. The size of ascidians depends on the species and age: there are half-meter giants, there are pygmies whose length does not reach 1 mm.

The body of the animal is covered with a gelatinous or cartilaginous tunic, which has supporting and protective functions. Tunica consists of tunicin, a carbohydrate similar in composition to plant fiber. The tunic, as a rule, is opaque and painted in bright colors, but it can be translucent and then the insides can be seen through it. In many species, the surface of the body is covered with folds, grains of sand, small pebbles and fouling materials (algae, hydroids, bryozoans and other sessile animals) so much so that it can be difficult to distinguish the ascidian from surrounding objects. The thickness of the tunic reaches 2-3 cm, although it is usually thinner. Under the tunic lies the muscle mantle. The muscles allow the sea squirt to compress the body. In this case, water is forcefully thrown out of the siphon. Ascidia uses this technique if it is frightened or chokes on a too large piece of food. The mantle lies freely inside the tunic and grows together with it only in the area of ​​the siphons. In these places there are sphincters - muscles that close the openings of the siphons.

Around the oral siphon there is a corolla of tentacles, sometimes simple, sometimes branched. Behind the mouth opening is the pharynx, which occupies almost the entire space inside the mantle. Gill slits are located along the walls of the pharynx, and an endostyle covered with glandular and ciliated cells runs from the dorsal side of the pharynx. It is the beating of the endostyle cilia that creates a constant flow of water through the mouth opening, gill slits, peribranchial cavity and from there through the cloaca to the outside. Between the rows of gill slits lie blood vessels. Passing through these cracks, water releases oxygen into the blood, and various small planktonic organisms and organic debris adhere to the mucus secreted by the glandular cells of the endostyle. The mucus is directed by the movement of the cilia of the epithelium into the short esophagus, and from there into the stomach, from which it enters the intestine. The intestine opens into the cloaca through the anus. Excreta is expelled from the body through the cloacal siphon.

The circulatory system of ascidians is not closed and consists of a heart, blood vessels and lacunae - cavities between organs that do not have their own walls. The nervous system of adult animals is extremely simple. The suprapharyngeal medullary ganglion is located between the siphons. From the ganglion, 2-5 pairs of nerves originate, going to the edges of the mouth, pharynx, intestines, genitals and to the heart, where there is a nerve plexus. There are no sensory organs, but the oral tentacles probably have a tactile function.

Ascidians are hermaphrodites: the same individual has both male and female gonads. Their ducts open into the cloaca. The eggs and sperm of one individual mature at different times, making self-fertilization impossible. Fertilization most often occurs in the peribranchial cavity, where the sperm of another ascidian penetrate with a current of water. Fertilized eggs pass through the cloacal siphon and develop into a larva. Sometimes the eggs develop in the peribranchial cavity and the larvae emerge.

Ascidian larvae are similar to a frog tadpole or sperm and are much more complex than the adult. Its length is several millimeters. The larval ascidian has a spinal tube lying above the notochord. The swelling at the anterior end of the tube corresponds to the brain of vertebrates. Among the sensory organs there are pigmented eyes and a statocyst - an organ of balance. The life of the larvae is short - from 2 hours to 5 days. All this time they do not feed, but can cover distances of up to 1 km, although most of them settle to the bottom not far from their parents. Having settled to the bottom in a suitable place, the larva attaches itself to the hard objects and begins to lead a sedentary lifestyle. And this is where the fun begins. The settled larva undergoes a significant simplification of the body structure. The tail retracts very quickly - in some species in 6-15 minutes. Remains of the notochord, statocysts and eyes resolve within a few days. When the larva develops into an adult animal, the entire posterior part of the neural tube disappears, and the brain vesicle, along with the larval sensory organs, disintegrates. The ganglion of the adult ascidian is formed from its dorsal wall, and the abdominal wall of the bladder forms the perinnervous gland. The body takes on a bag-like shape. This ends the transformation, as a result of which the animal turns out to be completely different from its own larva.

This is what a solitary ascidian looks and lives like. But there are many species of colonial sea squirts with to varying degrees integration of colonies. Even typical solitary ascidians often settle in large clusters and form large clumps, which gives them a number of advantages - during reproduction, feeding, and protection from enemies. In some already colonial species, individual zooid organisms are connected to each other only by the base, while they retain the typical characteristics of a solitary animal. More often than others, massive gelatinous colonies are found, the individual members of which are immersed in a common thick tunic. Sometimes a colony consists of zooids that have independent mouths, and their cloacal openings open into a common cloaca, forming long canals. In many species, zooids form a tight circle around a common cloaca (this group is called a cormidium), and those zooids that do not have enough space in the cormidium give rise to a new circle of zooids and a new cloaca.

It happens that the cormidium has a common colonial vascular system. The cormidium is surrounded by a ring blood vessel into which two vessels flow from each zooid. The vascular systems of individual cormidia communicate with each other, thus creating a complex vascular system that connects all zooids of the colony. Colonial ascidians form not only crusty growths on stones, but also form balls, cakes and outgrowths on legs that resemble mushrooms in shape. Colonies similar to sponges, soft corals and bryozoans are found. And there are species of such bizarre shape that it is difficult to recognize them as not only sea squirts, but simply a living creature.

This variety of colony forms is explained by the variety of methods of asexual reproduction by division and budding, in which new organisms receive the rudiments of all major organs from the mother. This is described in detail (but difficult) . In addition to asexual reproduction, ascidians can restore lost body parts. For example, the lower part of the body can build on the lost top part, can regenerate internal organs.

Ascidians feeding phyto- and zooplanton, as well as organic particles suspended in the water column. Few people liked them themselves - the tunic is tough and tasteless, and there is nothing else to eat in them. The bright color of many ascidians indicates their poor taste qualities. But a variety of symbionts like to settle in them: from microbes and protozoans, to fungi and algae, and marine animals of all systematic groups - coelenterates, worms, bryozoans, mollusks, crustaceans and even fish.

But people still eat ascidians. In the 19th century, they were eaten throughout the Mediterranean, but were always valued cheaply, and were mainly eaten by fishermen when sea squirts were caught in nets. I did not find any modern recipes, except for the only indication that ascidians are eaten raw: they cut the leathery tunic, take out the soft body and eat it whole. True, author this publication admits “that I tried all the seafood listed in the article, except for sea squirts. The desire to eat the hermaphrodite (sea squirts are hermaphrodites, which, you see, is also exotic food) was great, but I couldn’t overcome myself.” . UPD. (16/4/2009). However, a recipe was found in Chukotka. Those who read to the end will learn how to cook ascidian.

Their Red Sea sea squirt Eudistoma sp. in the mid-90s of the last century, an alkaloid was isolated that suppressed the development of leukemia in the laboratory. The substance was named eilatin in honor of our city. However, eilatin has not reached clinical use.

So people do not have any particular benefit from ascidians. But getting to know this interesting biological group makes you think about the life of not only biologists.

William Beebe(1877-1962), American traveler, writer, naturalist, famous inventor of the bathysphere, wrote in the book “Galapagos - the end of the world” :
“A small sac-shaped creature called a sea squirt. All she cares about is throwing eggs out from time to time. From the eggs emerge tadpole-like creatures with a brain, nerves, gills, and even a semblance of a spine; everything indicates that they, like fish, frogs or humans, have their own goal in development. But these creatures are an exception to the general scheme. The brain is destroyed, the notochord disappears, all signs of a “higher” purpose disappear. Now the fate of these animals is a peaceful passive existence... The noble origin, the untapped development opportunities inherent in them are forgotten! The timid response is only in the development of their own descendants..."

Felix Krivin , my twice compatriot, as always, is paradoxical:
“In their development, tunicates reached vertebrates and returned to invertebrates. That is, they began to develop backwards. Who said that we need to develop forward? If you have developed forward, you can develop backwards. It's time. True, it is not always possible to determine in which direction you are developing. When you sit in your own shell, sit and develop in your own shell, go and know in which direction you are developing: further forward or already backward.”

But Kiev resident Jey Timon on the pages your LiveJournal wrote:
“In its youth, at the larval stage, the ascidian is a free-swimming organism like a huge sperm. She has a head, a tail, a neural tube, a notochord and much more. Because tunicates are a phylum of chordates. Not bullshit. And then the larva firmly attaches to the substrate and switches to a sedentary lifestyle. The notochord disappears, the neural tube is reduced to a small nerve ganglion. And the tadpole itself turns into an adult ascidian - a motionless leathery sac with two holes, an entrance and an exit. And that’s one of the best metaphors for growing up in the world.”

Now let's go back to the kitchen. The only detailed recipe appeared on the Internet after I posted this article. It belongs to Ekaterina Vladimirovna, teacher kindergarten in the village Provideniya Chukotka Autonomous Okrug. It turns out that the Chukchi and Eskimos call sea squirt the word “upa” and upachat - they get it from the bottom of the sea with grappling anchors. In addition to the recipe, Ekaterina Vladimirovna wrote to me that she tried raw upa: in small quantity- as if I had gargled with an iodine solution. In general, she gradually adds boiled sea squirt to a salad like Olivier or even to a vinaigrette, it seems to get into the body and at the same time is not really felt (so that the children also eat). Boiled upa is slightly bitter. The indigenous population makes naval-style pasta from upa and pasta.