Phylum Nematoda

Phylum Nematoda

Nematodes (roundworms) are unsegmented, cylindrical (rather than flattened) worms, tapering at both ends, and are among the most numerically abundant animals on Earth, occurring as free-living forms in soil and water as well as parasites of plants and animals.

A defining structural feature of the phylum is the pseudocoelom — a body cavity that lies between the mesoderm and the endodermal gut but is not completely lined by mesoderm on every side (in particular, the gut lacks its own mesodermal covering). This distinguishes nematodes from the true coelomate phyla (such as Annelida) that appear later in the animal kingdom, even though nematodes otherwise show a protostome-type pattern in early development.

Nematodes possess a tough, flexible, non-cellular cuticle that covers the epidermis and is periodically moulted as the animal grows — this is a key structural difference from the epidermis of annelids (covered further in the following section), and it protects nematodes, especially parasitic ones, against the host's digestive enzymes and immune defences. …