Define the Features of Phylum Nematoda?

Phylum characteristics of Nematoda which are the world's most abundant multicellular organisms are as follows:

1. Nematodes occur in  all types of habitat  -  fresh water, marine waters and  terrestrial habitats and may be free living or parasitic on plants or animals.

2. Nematodes are usually long, cylindrical animals with tapered rounded ends though some parasitic forms become sac-like in shape.

3. Nematodes are,  bilaterally symmetrical, un segmented or superficially Segmented, triploblastic, pseudocoelomate  animals.

4. Nematodes are termed peseudocoelomate since their body cavity is a pseudocoelom which occurs between gut and body wall  and develops embryologically from the blastocoel of the blastula embryo.

5. Nematode body wall  is composed of (i) an outermost, strong, flexible, non-cellular cuticle  layer, (ii) an inner epidermis and (iii) an  innermost layer of longitudinal muscles.

6. The pseudocoel cavity is small and  is filled with the intestine and the reproductive system which consist of ovary and oviducts in  females or vas deferens and testes in males.

7. Digestive system consists of a straight tubular gilt which  includes mouth, muscular pharynx, pharyngeal glands and long intestine, that open posteriorly to the outside by a subterminal  anus.

8. Nematodes have specialized mouth parts, particularly  in  parasitic forms.

9. Nervous system  is simple, consisting of a circumpliaryngeal nerve around the pharynx that posteriorly gives rise to two nerves which run the length of the nematode, one on  the dorsal  (back) and one on  the ventral (belly) side.  A few tiny sense organs are located on  the head of the nematode.

10. Excretory system is present.  In  some nematodes, specialized excretory cells are found; in some examples canals may be present, while still  in  some others both excretory cells and canals may be found.

11. Flame cells are absent.

12. Respiratory and  circulatory systems are absent in nematodes.

13. Nematodes reproduce sexually. Most species have separate males and females  and so nematodes are called dioecious. Males are usually smaller than the  female of the species.