These are the popular round worms. The body is narrow and pointed at both the ends. There are no body segments. The body is covered by a thin cuticle. The body cavity is considered as a pseudocoelom. The alimentary canal is a straight tube. They reproduce sexually and the sexes are seperate. There are several free living soil nematodes. Others are parasites. (eg) Ascaris lumbricoides.
In subsequent Phyla the animals show following general characters
These are worm like animals. The body segments are rings externally. Internally the segments are seperated by septa. Externally the body is protected by a cuticle. Excretion and osmoregulation are acheived by ciliated tubules called nephridia. There is a central nervous system. The brain is formed of ganglia in the head region. The nerve cord is ventral in position. For the first time head formation or cephalization happens. These are bi-sexual and hermophroditic. The larva is called the trochophore.
This phylum includes three Classes, namely Polychaeta, Oligochaeta and Hirudinia. The polychaetes are marine worms. They have a distinct head. There are pairs of lateral projections called parapodia. The examples are Nereis (ragworms), Arenicola (lugworm).
Earthworms are included in the Class Oligochaeta. The Class: Hirudinia includes leeches. These are blood suckers and ectoparasites. They have well developed suckers for attachement at anterior and posterior ends.