Aschelminthes show following characters:

1. They are first invertebrates which posses body cavity. But they lack peritoneal linings  and mesenteries. These structures are found in more advanced animals. Therefore, the various internal organs lie free in the cavity. Such a cavity is called a pseudococlom or pseudocoel. Therefore, these animals are called Pseudocoelomates. The pseudococlom is often fluid filled. Or it may contain a gelatinous substance with mesenchyma cells. The pseudocoelom performs following functions:

(i)        This cavity is used for circulation.

(ii)         It helps in digestion.

(iii)It acts as an internal hydrostatic skeleton. It is used in locomotion.

2. Most aschelminths have a complete tubular digestive system. It extends from an anterior mouth to posterior anus. This complete digestive tract was first started in the nemerteans. It is characteristic of almost all other higher animals. The mechanical breakdown of food, digestion. absorption, and feces formation take place in it from anterior to posterior direction. Most aschelminths also have a specialized muscular pharynx. It is adapted for feeding.

3. Many aschelminths show eutely. In this case, numbers of cells are constant in the entire animal and in each given organ in all the animals of that species. For example, the number of body cells in cells in all adult Caenorhabditis elegans is 959 and the number of cells in the pharynx of every worm in the species is precisely 80.

4. Most aschelminths are microscopic. But some are meter in length.

5. They are bilaterally symmetrical, unsegmented, triplohlastic animals They are cylindrical in cross section.

6. Most aschelminths have an osmoreplator> system of protonephridia. There are more osmotic problems in the fresh water animals. Therefore, this system is best developed in freshwater forms.



7. No separate blood or gas exchange systems are present in them.

8. Some cephalization (head formation) is present in them. The anterror end contains a primitive brain, sensory orgins, and a mouth.

9. Majority of aschelminths are dioecious. Reproductive systems are relatively simple. Their life cycle is simple.

10. Cilia are absent on external surface. A thin. tough external cuticle is present. The cuticle may have spines and scales. These structures protect the animal. Sonic aschelminths shed this cuticle for growth. This process is called molting or ecdysis. Beneath the cuticle is a syncytial (multinucleate) epidermis. It actively secretes the .cuticle. Several longitudinal muscle layers liedieneath the epidermis.

11. Most aschelminths are freshwater animals. Only a few Live in marine environments. Many of the nematodes are parasite. The remaining aschelminths arc mostly free-living. Some rotifers are cilonial.

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