Ciliated worms

Free-living flatworms feed mainly on predators and crawl or swim. This is facilitated by the dermuscular sac and cilia. Flatworms are two-sided symmetrical animals.

Body shape and skin-muscular bag.

The body is flattened, oval or elongated. At the front end of the body, the senses are usually located. The mouth is on the ventral side of the body.

The body of the turbellarium is outside covered with a single-layer ciliated epithelium, and with age, the cilia are often lost, because of which the cells “bald”. Cilia are thought to contribute to the movement of the worm in space. The structure of the epithelium of different worms is not the same, in connection with which there are two main options. In the first of these, epithelial cells are clearly separated from each other. In the second, epithelial cells merge in their upper part, forming a common cytoplasmic nuclear-free layer. In the lower part of the cell form the sacs containing the nuclei, which descend below the basement membrane and maintain separation between them.

In the epithelium of turbellarians there are many single-celled glands of various shapes. They are scattered over the entire surface, but can form clusters. For example, the mucous glands are particularly numerous in the epithelium of the anterior part of the body, the mucus secreted by them, apparently, contributes to the attachment of the worm to the substrate. On the contrary, protein glands are concentrated around the edges of the body, it is believed that the secreted by them has toxic properties.

Among the epithelial glands, rhabditic cells are distinguished, which contain light refracting rods - rhabdites lying perpendicular to the surface. When irritated, rhabdites shoot out, where, when they come into contact with water, they quickly lick themselves and form protective mucus. Due to this, turbellarians are practically not eaten by other animals. Rabbit cells are located below the basement membrane, but they are associated with the overlying epithelial cells, into which the rhabdites enter. The muscular part of the skin-muscular bag is formed by several layers of muscle fibers. Outside there is an annular layer, under it is diagonal, the fibers of which run at an angle to each other, and the lowest - longitudinal. It is a combination of the integument and underlying layers of muscle fibers that form the skin-muscular sac. In addition to solid layers, muscle fibers also form bundles of dorsoventral muscles. They are directed from the dorsal (dorsal) part of the body to the ventral (ventral) part and, contracting, flatten the worm's body even more. Small worms swim with the help of ciliary epithelium, and large worms mainly due to wave-like contractions of the body. Worms crawl by contracting the muscles of the body or move with a “walking” movement, alternately attaching to the front or rear end of the body.

Digestive and Excretory Systems

The digestive system of ciliary worms is quite diverse within the class and varies from primitive — without an established intestine to relatively complex — with a branchy intestine . The main groups of ciliary worms differ primarily in the shape of the intestines.

A rather complex intestine is present in many branching turbellarians - polyclad (detachment Polycladida). The oral opening of them, as a rule, is closer to the posterior end of the body and leads to the folded pharynx, from which many blind branches of the endodermal intestine diverge radially.

In the three-branched suborder , or planarium - triclad ( Tricladida ) from the pharynx, located in the middle of the body, three branches of the intestine extend. One branch is directed to the front end of the body, and two - back. At the same time, food first enters the front outgrowth, and from there flows into the rear. In this way, nutrients are initially provided to organs located in the front of the body.

In the order of the rectus (Rhabdocoela), the pharynx is located at the anterior end of the body and the intestine is straight, unbranched.

In ciliary worms with intestines formed, the pharyngeal glands play an important role in the digestion of food. Many species have extraintestinal digestion. Planarias often attack rather large victims ( mollusks , crustaceans). They excrete digestive enzymes into the victim's body, and then suck half-digested food.

Despite a certain complexity of the digestive system, the absorption of food in turbellarians is largely due to intracellular digestion, that is, phagocytosis of epithelial cells of the midgut.

Most turbellarians are predators and feed on various small invertebrates . Finding the victim, the worm covers it with his body, and then swallows. For planarians for this is a retractable throat, which is ejected from the deep invagination of the outer cover - the pharyngeal pocket. If the size of the prey is too large and cannot be swallowed whole, the worm tears off the pieces with strong sucking movements of the throat, and then swallows them. However, small arthropods (for example, crustaceans) are covered with hard shell and planaria cannot break them. In this case, it releases digestive enzymes from the pharynx, which break down the victim’s tissues outside the worm’s body, after which the softened food is absorbed by the pharynx. Thus, in turbellarians, external digestion is also possible. Some turbellarians (planarians) have a peculiar ability to use "captured weapons". Scientists have found that when eating hydr planarians, stinging cells are not destroyed, but migrate to the integument of the worm and protect it.

Excretory system of protonephardial type. Usually there are one or two main channels, from which a lot of branching small tubules, ending in atrial cells - circtocytes, depart . At the posterior end of the body, the excretory canals open to the outside of the excretory pores. Protonephridia is most strongly developed in freshwater worms, which is associated with their additional function of osmoregulation. In the enteric planarians, amoeboid cells accumulate excreta excretory function. Excreted cells are removed through the skin.

Nervous system and sense organs

In the most primitive in this regard, intestinal turbellarians (for example, some convolut ), the nervous system is represented by a diffuse subepithelial nervous network, there is only its thickening at the anterior end of the body and the cerebral ganglion at the anterior end of the body (formed from four primordia in ontogenesis and the surrounding statocyst). In addition to the diffuse subcutaneous plexus, most turbellarians have paired nerve trunks, which originated as a derivative of this plexus, and annular bridges connecting them. This type of nervous system is an orthogon (lattice nervous system). In the enteric and other turbellarians with statocysts, the orthogon is connected to the endon brain , the nervous ganglion formed around the statocyst. In those with no statocyst turbellarium, an orthogonal-type brain is formed from thickened nerve trunks and ring commissures of the orthogon. The maximum number of orthogonic nerve trunks is 8, the minimum is 2. Some groups have additional deep lateral (lateral) trunks. For more advanced groups of turbellarians, orthogon immersion under the layers of muscles is characteristic. into the thickness of the parenchyma, reducing the number of nerve trunks and streamlining the commissures, moving most of the ganglion and motor neurons into the nerve trunks from the subcutaneous plexus.

The senses of the turbellarians are much better developed than those of other classes of flatworms. The skin of ciliary worms contains sensory cells with immobile long cilia that perform the function of the organs of touch and chemical sense. Some species have an organ of equilibrium - statocysts , located above or inside the brain ganglion. Over the entire surface of the skin are scattered cells with longer and immobile cilia, called sensilla . They are associated with processes of nerve cells. Sensilla provide perception of mechanical influences from the outside.

The most important role for turbellarians is played by the olfactory organs, since with the help of olfactory almost all of them prey on prey. In most species, the olfactory organs are olfactory pits located on the sides of the front end. They contain sensitive cells that send their axons to the front of the brain, glandular cells that produce mucus, and ciliated cells that create a stream of water that brings fragrant molecules into the hole.

Almost all turbellarians have inverted eyes. In most species, they do not have a lens and are not capable of differentiated (objective) vision, but in some ground-based planarians, the eyes have a complex structure, are equipped with a lens and, possibly, allow you to see objects. Usually there is one pair of eyes above the cerebral ganglion, but some (many planarians and most polyclad) may have several dozen eyes located in the brain area or bordering the entire front end of the body. Few representatives have 4 eyes or one unmatched eye. The pigment glass of the eyes is turned by the concave part towards the surface of the body; long, curved receptor (retinal) visual cells are immersed in it, at the extended ends of which there are light-sensitive structures. The light first passes through the body of the visual cells and only then enters their photosensitive part. Retinal cells are nerve in origin, so they have processes (axons), which together form the optic nerve, which is sent to the brain ganglion, where the obtained information is analyzed.

Reproductive System and Reproduction

Ciliated worms are hermaphrodites . In each individual there are female and male sex glands - the gonads. Many species have a complex system of genital ducts and additional glands.

Male gametes are formed in numerous small testes (in some tubulararia they may have only two) scattered in the thickness of the parenchyma . From each testicle there is a thin vas deferens channel, which flows into a larger paired canal - the vas deferens. When connected, the seed tubes form a ejaculating channel located inside the collective organ.

The female reproductive system consists of the reproductive glands - the ovaries, modified gonads - zheltochnikov and the female reproductive ducts. In many tubellaria, zheltochniki are not formed. From the ovaries, the ovules enter the oviducts (there are usually two of them), and the ducts of the zheltochnikas open up through which the yolk cells rich in nutrients enter. Together, the oviducts form an unpaired vagina, which opens into the genital cloaca.

In three-branched planarians, the reproductive system contains two testicles consisting of the seed sacs with the vas deferens. Paired vas deferens flow into the ejaculating channel, penetrating the copulatory organ - an analogue of the penis, which opens into the sexual foul spot - skin invagination, where the female ducts open. There are paired ovaries and oviducts extending from them. On the oviducts there are numerous glands - zheltochniki, producing yolk cells filled with yolk, necessary for feeding the developing eggs. Oviducts flow into the vagina, opening the sexual opening in the cloaca.

In the majority of tubobellaris, cross-fertilization occurs, in which the sexual partners pass on each other to the male sexual products, that is, the partners in turn act as male and as female. Typically, the copulatory organ introduces sperm directly into the genital cloaca, some have a small outgrowth in the genital cloaca — the copulatory bag, where the resulting seed is delivered. But in any case, internal fertilization takes place in ciliary worms. The fertilized egg, together with a group of yolk cells, is covered with a protective shell, and a complex egg is formed, which is characteristic of most flatworms.

The development of most ciliary worms is direct, that is, an organism resembling an adult animal emerges from an egg, but in some marine tubbellarii the development proceeds with metamorphosis . At the same time, the Mullerian larva, completely covered with cilia, due to which it swims, emerges from the egg completely different from the adult worm. For some time the larva swims as a part of plankton, after which it differentiates into a small worm.

Tubellaria can also reproduce asexually. At the same time, a transverse constriction appears on the body, gradually dividing the animal into two parts. Since some bodies are in the singular, the formed individuals subsequently complete the necessary parts.

In ciliary worms, especially in planarians, the ability to regenerate is pronounced. Small fragments the size of a tenth or even a hundredth part of the body of the planarian re-form the whole organism. For this ability of these animals is called "immortal under the knife of the operator" (there is also a little offensive expression for the relevant profession "immortal under the knife of the surgeon"). An amazing feature of planaria biology is its original response to adverse environmental conditions. For example, when there is a lack of oxygen or in the case of a strong increase in water temperature, the planarians themselves break up into pieces, which regenerate upon the occurrence of favorable conditions. This phenomenon is called autotomy .

• Beklemishev Century. N. Fundamentals of comparative anatomy of invertebrates. In 2 tons. - M .: Nauka, 1964.
• Ruppert E.E., Fox R.S., Barnes R.D. Zoology of invertebrates: functional and evolutionary aspects. - M .: Publishing Center "Academy", 2007. - T. 2. - P. 405-447. - ISBN 978-5-7695-3493-5 .