Table of contents:
- Place of tapeworms in scientific classification
- Species diversity and habitat of the parasite
- The structure and function of tapeworms
- Danger to humans
Last updated 3 March 2020 at 09:28 PM
Reading time: 6 minutes
Tapeworms, or cestodes, are a fairly large class of flatworms: about 3500 species have received their description. Such well-known representatives of tapeworms as echinococcus, pork and bovine tapeworm, broad tapeworm are dangerous to people. Infection with these worms occurs in various ways, including by eating infected and poorly processed meat or fish, unwashed berries, vegetables and fruits. For years, parasites can live in the human intestines, from time to time migrating to other organs of the body.
A notable feature of cestodes is the absence of their mouth and intestines, that is, their own digestive system. Everything about these parasites - from their physical structure to their mode of existence - is aimed at survival at the expense of another organism. The characteristics of the class of tapeworms necessarily include a description of their structure, life cycle and other distinctive features.
- 1 Place of tapeworms in scientific classification
- 2 Species diversity and habitat of the parasite
- 3 Representatives
4 Structure and function of tapeworms
- 4.1 Body
- 4.2 Nutrition
- 4.3 Excretory system
- 4.4 Nervous system
- 4.5 Reproductive function
- 4.6 Life cycle
5 Danger to humans
5.1 Similar articles
Place of tapeworms in scientific classification
Tapeworms are part of the Animal Kingdom, belong to the Flatworms type, and belong to this type as a separate class along with the Fluke (Trematodes) class. The main characteristic of tapeworms is their tape-like body. Because of him, they got their name. The international scientific name for this class of helminths is Cestoda. In the literature, you can find another name - tapeworms.
Species diversity and habitat of the parasite
Cestodes are highly organized creatures. Giving a general characteristic of tapeworms, we can say that their life cycle is quite complex and is carried out in such a way as to ensure the survival of each species in the best possible way.
All representatives of this class are helminths, that is, parasitic worms; there are no free-living among them. They use another body, the host's organism, for their own existence, and feed on it. Adults live in the small intestine. The larvae settle in tissues and body cavities. Throughout its life, the worm changes several hosts.
Tapeworms have a varied habitat. Ultimate hosts are vertebrates, both terrestrial and marine, while their intermediate hosts can be insects, crustaceans and molluscs, as well as other vertebrates.
The variety of sizes is another distinctive feature of tapeworms, representatives of which can be from a few millimeters to three tens of meters long. Thus, the largest species, the whale tapeworm Tetragonoporus calyptocephalus, grows up to 30 m. Bovine tapeworm - up to 20 m. The species parasitizing in small hosts are usually very small. For example, those that inhabit voles and lemmings are only 13 mm to 24 cm long.
Several orders form a class of tapeworms, whose representatives infect some invertebrates, all types of mammals, including humans, domestic and farm animals, and fish. For human health, the most significant are echinococcus, pork and bovine tapeworms from the order of true cestodes (cyclophyllid), as well as a broad tapeworm belonging to the order of false cestodes (pseudophyllid).
Tapeworms include amphilinids, gyrocotylids, and cloves parasitizing fish.
The structure and function of tapeworms
Once attached to the intestinal walls of the host, the worm is completely dependent on the host's nutrients. This is the way tapeworms live. The characteristics of their vital functions and structure are as follows: the digestive system is reduced, the nervous one is poorly developed. But on the other hand, the reproductive system is extremely developed, and this provides tremendous opportunities for survival.
The external structure of tapeworms can vary slightly, but, as a rule, an adult consists of the following parts:
- "Scolex", or head;
- short neck;
- strobila - a segmented body, consisting of a sequence of several segments, or "proglottids".
The scolex is equipped with four suction cups that attach it to the inner walls of the host's intestine. The head does not have a mouth, but there are two rows of hooks with which the parasite gnaws into tissue.
In the larval stage, the scolex has a simple structure and is called "protoscolex".
The cervix constantly produces proglottids, each of which has the ability to reproduce. There can be 3-4 of them, like in echinococcus, or reach the number of several thousand. One by one the segments ripen; the oldest and largest of them, located at the back of the body, is filled with eggs. This is the womb. After maturation, the uterine segment breaks off the rest of the parasite's body and goes "in free swimming". Some of them leave the host along with feces and enter the external environment, the other part leaves the anus on its own.
The body of the helminth is constantly being restored due to the constant formation of new segments.
Some worms, such as cloves and amphilinids, do not divide into proglottids.
The body is covered with nederma, a superficial epithelium characteristic of tissues with no boundaries between cells. Such an epithelium is called a tegument; it is characteristic of tapeworms and some other flatworms. The tapeworm tegument is provided with microtrichia - microscopic outgrowths. Through them, food is absorbed. Neoderma protects the helminth from the action of the digestive enzymes of the external organism.
The worms have circular and longitudinal muscles.
It cannot be said that the digestive system is well developed in tapeworms: the parasitic type of existence led to its degradation. Helminths do not have their own digestive tract, but the absorption of proteins, carbohydrates and other nutrients occurs through the entire surface of the body - the tegument. Digestion also occurs at the expense of the host and its digestive enzymes. The villi of the tegument adsorb these enzymes (peptidases, fasfatases, etc.), and they digest the food surrounding the helminth.
The excretory system of tapeworms is represented by two longitudinal excretory canals that run along the entire body. Small tubules, protonephridia, penetrate the entire parenchyma and flow into the large excretory canals. At the initial stage, the canals are connected in the last segment and form a kind of bladder. When a segment falls off, a new bladder does not form. The release of waste products occurs through special holes through which each of the channels opens outward.
The bundles of nerve fibers that form the paired ganglion are located in the head. The skin contains only tactile and receptor cells. This entire apparatus represents the nervous system of tapeworms, which is in its infancy. The worm perceives tactile sensations, smell and taste. The organ of vision is not developed.
Young segments have no genital organs. As it matures, each segment acquires a complete set of male and female organs. At this stage, the reproductive system of the worm is hermaphroditic. Male organs presented in a segment:
- vas deferens and seminal vesicles;
- antennae, or cirrus (sensitive projections-hairs of the posterior end of the body) represent the copulatory organ;
- genital opening.
- vitelline (oviduct);
- little body of Melissa.
After fertilization, which occurs through the genital pore, there is a reverse development of the genitals. In the last segments, only the uterus filled with eggs remains. Such proglottid segments are considered mature, and the helminth is considered sexually mature. Segments come off and go out with feces. When their walls break, the eggs spread into the surrounding space.
Fertilization can occur both within one segment, and with the fusion of two different proglottids belonging to the same individual.
For helminths of the cestode class, a complex life cycle consisting of several stages is characteristic. During its life, the worm changes several hosts.
- At the first stage, adult sexually mature individuals live in the small intestine of the host organism. There they feed, breed and produce eggs.
- Further, the eggs leave the host and fall on the soil or in water bodies, where the larvae form.
- At the third stage, once in the intermediate host, the larvae from the intestine with the flow of blood and lymph spread through its body. The larva settled in the muscles develops into a Finn - a vesicular worm, inside which several heads of the parasite float in the liquid. Finna, if she is lucky, enters the body of the main host and attaches itself with heads (scolexes) to the intestinal wall. The helminth begins to grow, develop and multiply again.
The main, or definitive host, as a rule, are carnivorous predators, which also include humans. Parasites enter the human intestine in the form of scolexes (parasite heads) when they eat raw or insufficiently fried infected meat or fish.
Intermediate hosts can be crustaceans, snails, or fleas. Sometimes the case is not complete without additional hosts: birds, fish, rodents or other mammals. They become infected by eating crustaceans or insects. And they, in turn, are hunted by predators - definitive owners.
Danger to humans
Like other mammals, humans suffer from tapeworms. The general characteristics of the diseases they cause are as follows:
- When infected with adult helminths, the intestines suffer, anemia develops;
- If the worm enters the body at the larval stage, various organs and tissues (liver, lungs, brain, subcutaneous tissue) are affected.
Bovine tapeworm enters the human body in the form of a Finn from the meat of cows, yaks and reindeer. Pork tapeworm is dangerous both in the larval stage and in the Finnish stage.
The broad tapeworm penetrates when eating raw fish or insufficiently salted pike caviar.
Echinococcus is the cause of very serious diseases. The eggs of the parasite enter the intestines through dirty hands, while swimming in polluted waters or with poorly washed forest berries.
Video fragment from the series "House Doctor" where they get a tapeworm from the stomach