Nematoda
The nematodes or roundworms (phylum Nematoda) are usually called roundworms and are found in freshwater, marine, and terrestrial environments. The head of a nematode is distinct. The rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, solid 'head-shields' radiating outwards around the mouth. The mouth has three or six lips, which often bear a series of teeth on their inner edges. An adhesive 'caudal gland' is usually found at the tip of the tail. The total number of nematode species has been estimated to be about 1,000,000 with over 16,000 that are parasitic. Instead of an endoskeleton or exoskeleton, Nematoda has a hydroskeleton which acts like the aqueous humor in our eye. This hydroskeleton keep the Nematoda in shape and prevents it from collapsing.
Characteristics of Nematoda:-
1)Bilaterally symmetrical, and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a pseudocoel, body fluid under high pressure.
4)Body possesses a through gut with a subterminal anus.
5)Body covered in a complex cuticle.
6)Has a nervous system with pharyngeal nerve ring.
7)Has no circulatory system (no blood system)
8)Reproduction normally sexual and gonochoristic.
9)Feed on just about everything.
10)Live just about everywhere, many species are endoparasites.
Two Major Classes of Nematoda
Adenophorea - lack of phasmids on the tails
Secernentea - no circulatory or digestive systems