A ciliated worm: a characteristic and a description of the class. Representatives of ciliated worms

A ciliated worm, or turbellaria (Turbellaria) refers to the animal kingdom , a type of flatworm, numbering more than 3,500 species. Most of them are free-living, but some species are parasites that live in the host's body. The sizes of individuals fluctuate depending on the habitat and nutrition characteristics. Some worms can be considered only under a microscope, others reach a length of more than 40 cm.

Parasites are almost all representatives of the type of flatworms. Ciliated worms are the only class that includes forms that freely inhabit the environment, but are predators.

Worms can be found in salty and fresh water bodies, in moist soil, under rocks, off the banks of rivers and lakes. Some live on the surface of the earth, others live below it. Small species live on the surface of the host's body, being parasites, but not causing him any particular harm. The most numerous and effective representatives of the class are planarians, which come in all possible colors (from black and white to brown and blue).

Description of the appearance of the ciliary worm

The class of ciliated worms is named so because the entire body of the worm is covered with small cilia, which ensure the movement of the animal and the movement of small animals in space. Ciliary worms move by swimming or crawling, like a snake. The shape of the animal body is flattened, oval or slightly elongated.

Like all representatives of flatworms, their body does not have an internal cavity. These are bilaterally symmetrical organisms, with the sensory organs in front and the mouth on the peritoneal part of the body.

Features of the ciliary cover

The ciliated epithelium is of two types:

• With cilia clearly separated from each other;
• With merged cilia in one cytoplasmic layer.

Cilia have not all representatives of the class of flat worms. The ciliated forms of the worms under the epithelial layer hide the secretion glands. Slime discharged from the front of the body helps the worm to attach and hold onto the surface of the substrate, as well as to move around without losing balance.

At the edges of the body of the worm are unicellular glands that secrete mucus with toxic properties. This mucus is a kind of protection of the animal from other larger predators (for example, fish).

Ciliated worms eventually grow bald, losing particles of epithelium, which resembles moult in animals.

Structure of the skin-muscular sac

The structure of ciliated worms is similar to the structure of all flatworms. The muscular organ forms a skin-muscular sac and consists of three layers of fibers:

• An annular layer located externally on the surface of the body;
• A diagonal layer whose fibers are at an angle;
• Longitudinal bottom layer.

Reducing, the muscles provide rapid movement and sliding of especially large individuals.

Digestive system

Some representatives of ciliated worms do not have a clearly defined intestine and are non-intestinal. In others, the digestive organs are represented by a whole system of branching canals, which supply nutrients to all parts of the body. It is the structure of the intestine that distinguishes the orders of ciliated worms. In addition to the non-intestinal (genus convolut), the ciliated worms are divided:

• Rectal (mesostomy);
• Branching (intestinal planaria, trikladidy).

The mouth of individuals with a branching intestine is located closer to the back of the body, the rectal to the anterior. The mouth of the worm is connected to the pharynx, which gradually passes into the blind branches of the intestine.

The class of ciliated worms has pharyngeal glands responsible for external (outside the body) digestion of food.

Selection system

The excretory system is represented by a number of pores in the posterior part of the animal's body through which unwanted substances are ejected through special channels. Small channels are connected to one or two main ones, adjacent to the intestine.

In the absence of intestinal secretions (excreta) accumulate at the surface of the skin in special cells, which after filling safely fall off.

Nervous system

Characteristics of ciliary worms include differences in the structure of the nervous system. In some types it is represented by a small network of nerve endings (ganglion) in the anterior part of the body.

Others have up to 8 paired neural trunks with a large number of neuronal branches.

The sensory organs are developed, special immobile cilia are responsible for the tactile function. Some individuals have a developed sense of balance, for which the special body of the statocyst, represented in the form of subcutaneous vesicles or pits, is responsible.

Perception of movements and irritating actions from the outside takes place through sensilla - immobilized cilia on the entire surface of the body.

In worms with the presence of a statocyst, an orthogon connected to it is formed - a system of brain channels of the lattice type.

Developed sense of smell and vision

The ciliated worm has olfactory organs, which play an important role in its life as a predator. It is thanks to them that the turbellarians find food. On the sides of the posterior and anterior ends of the body are pits, which are responsible for the transfer from the outside of signals and molecules of smelling substances to the brain organ.

Worms do not have sight, although there is a suggestion that some particularly large terrestrial species are able to visually distinguish objects, they have a formed lens. Although the eyes, and in most cases several dozen pairs and unpaired eyes are located near the worm in the region of the brain ganglions on the front surface of the body.

Light entering the visual sensitive retinal cells on the concave parts of the eyes provokes the production of a signal that is delivered to the brain for analysis by means of nerve endings. Retinal cells are similar to the optic nerve, which transmits information to the brain ganglia.

Breathing animals

The characteristic of the class of ciliated worms differs from the type of flatworms in that free-living individuals are able to absorb oxygen-breathe. After all, most flatworms are anaerobes, that is, organisms living in anoxic environment.

Breathing is vital and occurs through the entire surface of the body, which absorbs oxygen directly from the water through a multitude of microscopic pores.

Feeding of ciliated worms

Most of these animals are predators and many of them have an external digestive system. Attaching mouth to the potential victim, the worm allocates a special secret, produced by the pharyngeal glands, which digests food from the outside. Already after this, the worm sucks out the nutritious juices. This phenomenon is called external digestion.

Feed on the type of flat worms is the class of cilia mainly small crustaceans and other invertebrates. Not being able to swallow and crack the shell of a large size of crustaceans, the worms secrete a special mucus filled with enzymes. It softens the victim, practically digesting it, and the worm then simply sucks the contents of the shell.

The presence of teeth to worms replaces the pharynx, with which they swallow food whole. If the victim is of large size, then the worm sharply sucked the mouth of the mouth from it in a small piece, gradually absorbing all the prey.

Reproduction

The class of ciliated worms is represented by hermaphrodites, which have both male and female gonads. Male cells are found in the testes. Special seed canals emerge from them, delivering the sperm to the place of encounter with the eggs.

Female genital organs are represented by ovaries, from which the ovules are sent to the oviducts, then into the vagina, and then into the formed sexual cloaca.

Sexual fertilization occurs in a cross way. Worms alternately impregnate each other, alternately injecting sperm through the copulatory organ, resembling a penis, into the hole of the sexual cloaca.

Seed fluid fertilizes eggs and an egg covered with eggshells is formed. Out of the body of the worm, eggs emerge, from which the individual hatch, with the appearance already resembling an adult worm.

Only in turbellarians (type flat worms the ciliary class) from the egg appears a microscopic larva, similar to an adult, that swims with cilia along with plankton until it grows up and transforms into an adult worm.

These worms can also reproduce asexually. At the same time, a waist appears on the body of the worm, which gradually divides it into two equal parts. Each part becomes a separate individual, which grows the organs necessary for life.

Amazing ability to regenerate

Some representatives of ciliated worms, for example, planaria, are able to re-create damaged areas of the body. Even pieces of a body the size of a hundredth of a whole individual can grow into a new full-fledged worm.

Planaria from the squad three-branched thus learned to survive in adverse environmental conditions for it. With a significant increase in water temperature, with a shortage of oxygen, worms spontaneously break into pieces to regain recovery by regeneration, when external conditions return to normal.

A ciliated planar worm is the largest representative of the class that lives in water bodies. The predator feeds on small invertebrates. Themselves the food for fish worms do not become due to the presence of glands that release toxic substances.

Parasites

Parasitic ciliated worms include:

• Dark-tusles, which live on the skin of freshwater invertebrates and turtles, laying eggs on the surface of the host's body. Temncephals of small size (up to 15 mm), their body is flat, there are several tentacles. The ciliated worm is a hermaphrodite and lives mainly in the Southern Hemisphere.
• Udonlidy - previously referred to flukes, but now they are allocated to the detachment of ciliated worms. They have a body in the form of a cylinder and a small size (up to 3 mm). With the help of a sucker, they attach to crustaceans, which in turn parasitize on the gills of large marine fish.

Some species of turbellarians live only in the waters of Lake Baikal, which is due to the uniqueness of its waters. Most ciliated worms not only do not cause harm, but are also an integral part of their habitat. Destroying small mollusks, they keep a population of invertebrates under control , preventing it from increasing to an incredible size.