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.