Flat worms. Some representatives

Twelve and a half thousand kinds of flatworms are known. They are all divided into three classes. The first class includes turbellarians (cilia), to the second - flukes, to the third - ribbon flat worms.

They appeared about during the Proterozoic. These were the first three-layer animals with bilateral symmetry. As a rule, flat worms predators feed on protozoa. Animals live in water.

Flat worms-parasites appeared somewhat later. They are characterized by altered individual organs of digestion, senses. These flat worms live in the body of animals, humans or on the body of fish. They cause poisoning, severe illnesses, exhaustion, as well as severe damage to the blood system. The medical significance of flatworms was first formulated by Skryabin. The academician developed a whole science - helminthology. A distinctive feature of parasitic species is the availability of various adaptations (idioadaptations). So, these animals show suckers, hooks. They are characterized by great fertility.

Ciliated flat worms

This class has about three thousand species. They dwell mainly in fresh and marine water bodies. Rarely can they be found in the soil.

One of the representatives of ciliated worms is the milk (white) planaria. It lives in fresh water. Its body has a leaf-like shape, a pointed back and an enlarged anterior end. The length of the planaria is of the order of one to two centimeters.

The reproduction of this species is carried out both asexually and sexually. In the first case, there is a transverse division of the body into two parts.

The reproductive system is hermaphroditic and is rather complicated. Fertilization is cross-fertilized.

Flukes

This class of flatworms is represented by parasitic forms. They dwell in the organs of man and vertebrate animals, and also occur in invertebrates. There are about four thousand species of these worms.

One of the representatives is the liver trematode. The size of its leaf-shaped body is of the order of three to five centimeters. Adult individuals live in the bile ducts of the liver, the gallbladder in humans and herbivores. On the ventral side and the front of his body is the abdominal and oral sucker. With the help of these devices, the fluke can be held in the body of the host. It should be noted that these beings have rather weak sensory organs. Only the larvae floating in the water have eyes.

Flukes are a very prolific species. From one individual within a week can reach up to a million eggs. Their further development is carried out only in water. In the pond of eggs, larvae are formed, which must enter the intermediate host.

Asexual division of larvae occurs in the body of a mollusk (intermediate host). As a result of this multiplication, a whole generation of larvae is formed, the structure of which resembles the structure of adult individuals. After that, they leave the body of the mollusc and settle on coastal algae. Cysts are formed. Within them, the larvae retain their viability for a certain time. Together with food or water, parasites enter the body of a person or animal.

Liver flukes are hermaphrodites. Sexual reproduction takes place in the final body of the host.

Tape worms

This class has about three thousand species of parasitic forms. Inhabit almost everything only in the intestines of man and various vertebrates.

Tape worms have a ribbon-shaped body consisting of a cervix (a site in which immature joints bud off), heads and segments, in each of which a hermaphroditic system of reproduction is repeated. Mature segments are located in the posterior region of the body and are filled with fertilized eggs.

Tape worms can have a length of 0.5 millimeters to ten meters.

The representative of the class is bovine tapeworm. Its growth comes from the neck. In adulthood, bovine tapeworm parasitizes in the small intestine in humans, attaching to the walls of the organ with four suckers.