Various classes such as Flatworms are of great interest to science. And the point is not only 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 illnesses in humans and domestic animals. These creatures are combined by zoologists in one group with free-living ciliary worms - in the type Plathelminthes (flatworms). Their general characteristic is presented in our article. The most famous of them are precisely parasitic forms that provide an excellent example of the alternation of generations characteristic of a parasitic way of life. Our characterization of the type of flatworms begins with the structural features of these organisms.
Structural Features of Flatworms
For flukes, the presence of hooks and suction cups, a complex reproductive system, and reduced sensory organs are typical. And tapeworm segments are even devoid of intestines because they live in ready-to-eat foods. Essentially, these animals are just bags filled with eggs. Flatworms have a rather simple structure based on bilateral symmetry. Its general characteristic is as follows: the right and left halves of the body are mirror-like. Free-living flatworms have a flattened body shape and only one hole in the intestine is the mouth. These organisms lack the circulatory system. First, let's talk about 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 the intestinal cavity, in flatworms are separated by a third cell layer - the mesoderm, from which muscle tissue and genitals develop. The emergence of organ systems is a further step forward compared with the organization of intestinal. Most free-living flatworms are aquatic organisms. They move with the help of muscle contractions or movements of the cilia that cover their body. Predatory flatworms use the pharynx (the organ that connects the mouth opening to the intestines) when feeding: they press it to the prey and tear off pieces of food due to muscle contractions, which then enter the intestines. Undigested food leftovers return to the throat and out.
Flatworms, the structure of which we briefly examined, is the first group of animals in which a real excretory system appears. It is represented by two excretory tubes uniting the “fiery cells” and excretory pores opening at the posterior end of the body. “Flaming cells” got their name because of the constantly growing bundles of cilia inside them, which regulate the water balance.
We invite you to get to know a separate representative of a type such as flatworms. A general description and a photo of it will help you introduce this interesting organism.
Procerodeslittoralis
The free-living flatworm Procerodeslittoralis reaches a length of 2 cm and lives on rocky coasts. It belongs to the class Turbellaria, most of which are aquatic. The ribbon-like body makes it easy to diffuse oxygen and end products of metabolism, which is very important for an animal without a circulatory system. This is its general characteristic. We will continue to consider the type of flatworms, proceeding to a description of the nervous tissue and visual organs.
Nerve tissue, visual organs of free-living worms
The accumulation of nerve tissue at the anterior end of the flatworm body forms the brain, to which the nerve cords from two primitive eyes go. However, most flatworms shun the light and seek food through chemoreceptors. In experiments, they quickly respond to the smell of food that spreads in water.
Ciliary flatworms have the following structure. Their eyes are located on the head end of the body, above the brain, from which a pair of nerve strands depart. The pharynx, capable of turning inside out, opens into the branching intestine. All species are hermaphrodites, having both ovaries and testes. The genital opening leads to the genital cesspool, in which the copulatory organ lies . Passing through the oviduct, the eggs feed on secretions of the vitelline.
Flatworm flukes: lifestyle
The simplest form of parasitism is ectoparasitism, that is, life on the outer covers of the host. Some flukes adhere to it. For example, Gyrodactylus is attached with hooks and suction cups to the gills of fish, on which it feeds. But for humans, internal parasitic flukes are more important, such as Schistosoma (pictured below), which causes schistosomatosis, and liver fluke (Fasciola), which usually affects the bile ducts and sheep bladder.
Adult blood flukes of both sexes, parasitic in humans, live in the blood vessels of the intestines, causing inflammation and internal bleeding. Eggs are secreted out through the bladder or intestines and, once in the water, give a free-floating larva that penetrates the tissues of the cochlea. In the course of further development, other larvae appear that fall into the water and infect humans, invading the blood through the skin.
Hepatic fluke parasitizes in the bile ducts and bladder of cows and sheep. She is very prolific: produces over 40,000 eggs that leave the host with feces. Free-floating larvae emerge from the eggs and end up in the intermediate host, the freshwater mollusk. After a series of stages, a tiny caudate resemblance of an adult fluke appears, which extends through the walls of the mollusk body, climbs onto the stem of a plant and remains there until it is eaten by a sheep.
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 with a new owner. 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 dies, the remaining can survive by finding a new owner.
Types of hosts
The animal on which the adult trematode parasitizes is called the "ultimate" host. To facilitate the transition of the parasite from one final host to another, there is one or more intermediate. Schistosome uses only one intermediate host. But the Chinese hepatic fluke (Clonorchis, shown in the photo above) - two: shellfish and fish. In the organism of intermediate hosts, the parasite undergoes special larval stages, during which it increases its number through asexual reproduction.
Tapeworm lifestyle
In tapeworms, a further simplification of the body structure is observed. The digestive system is absent, since a parasite immersed in the semi-digested contents of the intestines of the host can only absorb the substances it needs. The head of the worm, armed with hooks and suction cups, is attached to the host. The rest of the body is simply a series of individual segments, or proglottids, that form in the upper part of the tape, freely hanging in the lumen of the intestines of the host.
Apart from the excretory tubules and neural cords, the tapeworm segments are, in essence, reproductive organs. Branched vas deferens fall into the ejaculatory canal, which, together with the vagina, opens through the genital opening. The eggs, leaving the ovaries, are supplied with secretions of the shell glands and the zheltochnik. The eggs are stored in the uterus.
Stages of development of tapeworms
Fertilized lentecian eggs, equipped with yolk and surrounded by a protective membrane, accumulate in the uterus of the worm. Mature segments come off and, together with the feces of the host, are secreted out. Like flukes, tapeworms have one or more intermediate hosts. For example, in the broad ribbon (Diphyllobothriumlatum), which is found in the intestines of humans, dogs, and cats and reaches a length of more than 9 m, two intermediate hosts, and one in the tapeworm (Taenia solium).
Wide Ribbon Development Cycle
The characteristic of a flat worm of the form wide ribbon (in the photo above) is as follows. It has two intermediate hosts - the Cyclops and one of the many freshwater fish in Europe, America and the Far East. An adult worm lives in the human intestines and can reach several meters in length. End segments come off and come out with feces, carrying up to 13 million eggs. daily. In water, an embryo hatches from an egg, which the cyclops eats. In it, the embryo develops into the first larva. If the fish eat the cyclops, a second larva is formed, which is introduced into the fish tissue. If a person eats this infected and insufficiently cooked fish, it is freed. Using tiny hooks, the larva attaches to the intestinal wall of a person and in 3 weeks turns into an adult form. The cycle repeats.
Pig Chain Development Cycle
In conclusion, you are invited to characterize a flatworm of another species. Consider the development cycle of pork tapeworm. Its eggs, secreted out with human feces, will develop only if they enter the pig’s intestines, where the digestive juices dissolve the egg shell and the embryo comes out, armed with six hooks. He drills the wall of the intestine and enters the bloodstream, through which he gets to the muscles. Here he incites, forming a Finnish vesicle, and remains at this stage until the infected meat is eaten by a person. And the cycle of development of the parasite will repeat.
So, we briefly examined the organisms of interest to us, given their general characteristics. The type of flatworms causes a lot of problems to humans, so scientists are developing new means of dealing with them.