Platyhelminthes

The group of the plathelminthes (in Latin Platyhelminthes , of the Greek platus , “broad” and helmins , “worm”) or towards dishes contains mainly worms which are animals lengthened without distinct head and appendix. They have a bilateral Symétrie and are Acœlomate S (they do not have a general cavity: neither cœlome nor pseudocœlome). The digestive tract has an opening (stops). This group is composed of surroundings 20000 species and comprises four classes which correspond to adaptations to a precise medium, thus one a:

  • Turbellarié S like the Planaire;
  • Monogène S which is parasitic S sailors or of fresh water;
  • Trématode S which is parasites like the Liver fluke;
  • Cestode S like the Tapeworm or Tænia (tapeworm), or many the cestodes which can infect the animals, of which the birds which can transport them from a continent to the other.

The flat worms, except some species posing of the problems important for the human health or animal, or used as model in laboratories are still badly known. Some collections naturalists of reference preserve of them samples collected in the world, of which that of the MHNG (Muséum of natural history of Geneva) considered to contain approximately 25% of the types of cestodes world, with more: 30000 preparations.

The Planar , one turbellarié

Interactions with the environment

Protection

The planar one has a epithelium unistratifié with glands which secrete a Mucus which confers in particular a protection to him against the Prédateur S, while facilitating its displacements.

Sensitivity

The planar one has photoreceivers which are the ocelles ones, their information are integrated in the cerebroid ganglia located just in lower part.

Locomotion

The planar one has circular, longitudinal and transverse muscles which enable him to contract its interstitial medium, and thus to move. This displacement is less effective than at the nematodes which have a Hydrosquelette.

Nutrition

The planar one has a Pharynx, i.e. a muscular structure used at the same time for ingestion and the evacuation of food. The pharynx leads to a digestive tract plugs with three principal ramifications which are distributed in all the body of the animal.

The rudimentary system excretor consists of Protonéphridie S. Certaines cells have cilia which cause a water call. As the planar one is far from thick, metabolic waste can also be eliminated by simple diffusion.

The plathelminthes do not have a breathing apparatus. The Oxygen, necessary to the cellular Metabolism, passes through thin the tegument of the animal.

Reproduction

The plathelminthes are generally hermaphrodites, i.e. they have at the same time of the male and female reproductive bodies. At the planar one, the testicles are numerous and are distributed in all the animal. The females have ovaries made up of glands ovariennes and vitelline glands which secrete the Vitellus, nutritive concerning eggs.

The plathelminthes can also reproduce in an asexual way and they are endowed with regeneration.

The liver fluke, a trematode

The animal has a mouth in its former part and also two suction cups, former and the other posterior one. As for the planar one, the ditch is not regionalized. The ditch has as a final host the sheep where it is in its bile ducts. The cycle of the ditch is complex. Fecundation is crossed and much eggs are laid, which makes it possible to increase the probability of meeting with the final host. The intermediate host is an Gastropod (Limnée for example) which is infected by larvae miracidium, attracted by secretions of the Gastropod. Inside this host the Ditches reproduce by asexual multiplication (stages rediae). Then the Gastropod emits cercaires which can enkyster and to give métacercaires and to survive several weeks until ingestion by the final host.

The tapeworm or tapeworm, a cestode

Interactions with the environment

The tapeworm is a parasitic intestinal of the man, involving Tæniasis. It is not digested because in the skin there are invagenocytes called microtrides with at the base of the épidermiques cells. There is a syncytium with a layer of cells which result from the fusion of mesoblastic cells and which is renewed regularly. There is a néoderme which is protected from digestion. There is a dynamic protection.

Nutrition

The tapeworm has neither digestive system, neither breathing apparatus, nor circulatory apparatus. The nutrients, oxygen and waste diffuse through the tegument.

Function of reproduction

The final host of the tapeworm is the man. The parasite is fixed in our digestive tract at the level of the duodenum thanks to a body specialized in anchoring, the scolex. With the back a zone of intense cellular multiplication is which gives a succession of rings hermaphrodites. There is a reproduction between various rings. The eggs are then eliminated with the excrements from the host and the eggs will develop then in the conjunctive walls of the muscles of the pig or ox, if the latter eat the excrements. The hatched eggs will fix themselves in order to remake a cycle.

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