Flatworms: general characteristics and structure

Different classes of the type Flat worms are of great interest for science. And it's not just that these organisms are interesting in themselves. Many of them are also very dangerous. Flukes and tapeworms are two groups of parasitic animals that often cause serious human and pet diseases. These creatures are united by zoologists into one group with free-living ciliated worms - in the type Plathelminthes (flatworms). A general characteristic of them is presented in our article. The most famous of them are parasitic forms, which give a fine example of the alternation of generations, characteristic of the parasitic way of life. Our characteristic of the type of flatworms begins with the structural features of these organisms.

Features of the structure of flatworms

For flukes, the presence of hooks and suckers, a complex reproductive system and reduced sensory organs are typical. And segments of tapeworms are devoid of even the intestines, since they live in ready-to-eat foods. In essence, these animals are just bags filled with eggs. A fairly simple structure based on bilateral symmetry has flat worms. Its general characteristic is the following: the right and left halves of the body are mirror-like. Free-living flatworms have a flattened body shape and only one opening in the intestine is oral. The circulatory system these organisms are deprived. Let's first describe free-living species, describing the type of flatworms. Their general characteristics are presented below.

Free-living flatworms: nutrition, movement and excretion

Ectoderm and endoderm, characteristic of coelenterates, in flatworms are separated by a third cellular layer - the mesoderm, from which muscle tissue and genital organs develop. The appearance of organ systems is a further step forward in comparison with the organization of coelenterates. Most free-living flatworms are aquatic organisms. They are moved by means of muscle contractions or movements of cilia that cover their body. Carnivorous flatworms are used to feed the pharynx (the organ that connects the oral opening with the intestine): they press it to extract and, through muscle contractions, tear off pieces of food that are then fed into the intestine. The undigested remnants of food return to the pharynx and are discharged outside.

Flat worms, whose structure we briefly examined, is the first group of animals that have a real excretory system. It is represented by two excretory ducts that unite "fiery cells" and are opened at the posterior end of the body by excretory pores. "Flame cells" got their name because of constantly fluctuating in them bundles of cilia, which regulate the water balance.

We suggest you get to know a separate representative of this type, such as flatworms. General characteristics and photos of it will help you to imagine this interesting organism.

Procerodeslittoralis

The free-living flat worm Procerodeslittoralis reaches a length of 2 cm and lives on stony sea coasts. He belongs to the class Turbellaria, the majority of whose representatives lead an aquatic life. The ribbon-like body makes it easy to diffuse oxygen and end products of metabolism, which is very important for an animal devoid of the circulatory system. This is its general characteristic. The type of flatworms will continue to be considered, going on to describe the nervous tissue and visual organs.

Nervous tissue, visual organs of free-living worms

The accumulation of the nerve tissue at the anterior end of the body of the flat worm forms the brain, to which the nervous strands from the two primitive eyes go. Nevertheless, most flatworms avoid light and find food with chemoreceptors. In experiments, they react quickly to the smell of food spreading in the water.

Ciliated flat worms have the following structure. Their eyes are located at the head end of the body, above the brain, from which a pair of nervous strands leave. A pharynx, capable of turning inside out, opens into a branching intestine. All species are hermaphrodites, having both ovaries and testes. The genital opening leads to the genital cloaca, in which the copulatory organ lies. Passing through the oviduct, the eggs are fed by secretions of vitellaria.

Flatworms-flukes: a way of life

The simplest form of parasitism is ectoparasitism, that is, life on the outer covers of the host. Some trematodes adhere to it. For example, Gyrodactylus is attached using hooks and suckers to the gills of fish, on which it feeds. But for a person, internal parasitic trematodes, such as Schistosoma (pictured below), causing schistosomiasis, and the liver fluke (Fasciola), which usually infects the bile ducts and the sheep's bladder, are more important to humans.

Parasitizing in man, adult blood flukes of both sexes live in the blood vessels of the intestine, causing inflammation and internal bleeding. Eggs are released outward through the bladder or intestine and, after getting into water, give a free-floating larva that penetrates into the tissues of the cochlea. In the course of further development, other larvae appear, which enter the water and infect humans, penetrating into the blood through the skin.

Hepatic fluke parasitizes in bile ducts and a bubble of cows and sheep. It is very prolific: produces over 40,000 eggs that leave the host with feces. Free-floating larvae emerge from the eggs and enter the intermediate host, the freshwater mollusk. After a number of stages, a tiny, caudate semblance of an adult flukes appears, which emerges through the walls of the body of the mollusk outward, climbs on the stem of a plant and stays there until the sheep eat it.

The needs of internal parasites are quite simple. Therefore, their locomotor and digestive systems, as well as sensory organs, are reduced. The reproductive system is better developed to provide at least part of the offspring a meeting with the new host. Therefore, in all trematodes, the reproductive organs are usually very massive and a large number of germ cells are formed in them. Even if 99% of the offspring die, the survivors can survive by finding a new master.

Host types

The animal on which the adult trematode parasitizes is called the "final" host. To facilitate the transition of the parasite from one final host to another, there is one or more intermediate ones. Schistosome uses only one intermediate host. But the Chinese hepatic fluke (Clonorchis, pictured above) is two: shellfish and fish. In the body of intermediate hosts, the parasite passes through special larval stages, during which it increases its numbers by asexual reproduction.

Lifestyle of tapeworms

Tapeworms further simplify the structure of the body. The digestive system is absent, since the parasite, immersed in the half-digested contents of the host's intestine, remains only to assimilate the substances necessary for it. The head of the worm, armed with hooks and suckers, is attached to the host. The rest of the body is simply a series of individual segments, or proglottids, that form at the top of the ribbon, freely hanging in the lumen of the host's intestine.

Apart from the excretory tubules and nerve strands, the segments of the tapeworm are, in effect, the organs of reproduction. Branched vas deferens enter the ejaculatory duct, which, together with the vagina, opens with a genital opening. Eggs, leaving the ovaries, are provided with secretions of the shell glands and vitellaria. Eggs are stored in the uterus.

Stages of tapeworm development

Fertilized egg lentets, supplied with yolk and surrounded by a protective shell, accumulate in the womb of the worm. Mature segments come off and, together with the feces of the host, are released outward. Like the flukes, tapeworms have one or more intermediate hosts. For example, in a wide ribbon (Diphyllobothriumlatum), which occurs in the human intestine, dogs and cats and reaches a length of more than 9 m, two intermediate hosts, in the pork tapeworm (Taenia solium) - one.

The cycle of development of a wide ribbon

The characteristic of a flat worm is a kind of wide ribbons (pictured above) next. It has two intermediate hosts - Cyclops and one of the many freshwater fish of Europe, America and the Far East. An adult worm lives in the human intestine and can reach several meters in length. The terminal segments come off and come out with feces, carrying up to 13 million eggs. daily. In the water, an embryo hatches from the egg, which the cyclope eats. In it, the fetus develops into the first larva. If the Cyclops eat the fish, a second larva forms, which is introduced into the fish tissue. If this infected and not enough cooked fish is eaten by a person, it is released. Using tiny hooks, the larva is attached to the wall of the intestine of a person and in 3 weeks turns into an adult form. The cycle is repeated.

Cycle of pork chain development

In conclusion, you are offered a characteristic of a flat worm of another kind. Consider the development cycle of pork chain. Eggs that are released outward with human feces will develop further only if they enter the intestine of a pig, where digestive juices dissolve the egg shell and from it an embryo armed with six hooks will emerge. He will drill the wall of the intestine and enter the bloodstream, through which he will reach the muscles. Here it is encysted, forming a vesicle-Finn, and remains at this stage until the infected human meat is eaten. And the cycle of parasite development will repeat.

So, we briefly examined the organisms of interest to us, given their general characteristics. The type of flatworms poses a lot of problems for a person, so scientists are developing all new ways to deal with them.