About twelve and a half thousand species of flatworms are known. All of them are divided into three classes. The first class includes turbellaria (ciliary), the second - flukes, the third - flat tapeworms.
They appeared approximately during the Proterozoic. These were the first three-layer animals with bilateral symmetry. As a rule, flat predator worms feed on protozoa. Animals live in water.
Flat parasitic worms appeared a little later. They are characterized by altered individual digestive organs, senses. These flatworms live in the body of animals, humans, or on the body of fish. They cause poisoning, serious illness, exhaustion, as well as severe damage to the blood system. The medical significance of flatworms was first formulated by Scriabin. The academician has developed a whole science - helminthology. A distinctive feature of parasitic species is the presence of various devices (idioadaptations). So, in these animals suckers, hooks are found. They are also characterized by great fecundity.
Ciliary flatworms
This class has about three thousand species. They live mainly in fresh and marine bodies of water. Rarely can they be found in the soil.
One of the representatives of ciliary worms is milk (white) planaria. It lives in fresh water. Her body has a leaf-shaped shape, a pointed rear and widened front end. The length of the planaria is about one to two centimeters.
Reproduction of this species is carried out both asexually and sexually. In the first case, the body is transversely divided into two parts.
The reproductive system is hermaphroditic and complex. Cross fertilization.
Flukes
This class of flatworms is represented by parasitic forms. They live in the organs of man and vertebrates, and are also found in invertebrates. There are about four thousand species of these worms.
One of the representatives is the hepatic trematode. The size of its leaf-shaped body is about three to five centimeters. Adult individuals live in the bile ducts of the liver, the gall bladder in humans and herbivores. On the abdominal side and front of his body is the abdominal and oral sucker. Using these devices, the fluke manages to stay in the host's body. It should be noted that these creatures have rather poorly developed sense organs. Only larvae floating in the water have eyes.
Flukes are a very prolific species. Up to a million eggs can be released from one individual within a week. Their further development is carried out only in water. In a pond of eggs, larvae are formed that must enter the intermediate host.
Asexual division of larvae occurs in the body of the mollusk (intermediate host). As a result of this reproduction, a whole generation of larvae is formed, the structure of which resembles that of adult individuals. After that, they leave the body of the mollusk and settle on coastal algae. Cysts are formed. Inside them, the larvae remain viable for a certain time. Together with food or water, parasites penetrate the body of a person or animal.
Hepatic flukes are hermaphrodites. Sexual reproduction is carried out in the final body of the host.
Tapeworms
This class has about three thousand species of parasitic forms. Almost all live only in the intestines of humans and various vertebrates.
The tapeworms have a ribbon-like body consisting of a neck (a section in which immature segments bud), a head and segments, in each of which the hermaphroditic reproduction system is repeated. Mature segments are located in the back of the body and are filled with fertilized eggs.
Tapeworms can have a length of 0.5 millimeters to ten meters.
The representative of the class is bull tapeworm. Its growth comes from the neck. In adulthood, bovine tapeworm parasitizes in the human small intestine, attaching itself to the walls of the organ with four suction cups.