Annelids are worm like animals. Their body is characterized by external rings and internal septa. Excretion and osmoregulation are controlled by nephridia. Their body is surrounded by a cuticle. They have a central nervous system and the brain is formed of ganglia in the head region. Head formation or cephalization occurs. They may be bisexual or hermaphroditic.
This phylum includes three Classes, namely Polychaeta, Oligochaeta
and Hirudinia. Examples are earthworms, leeches, ragworms.
Nereis (ragworms), Arenicola (lugworm).
Earthworms are included in the Class Oligochaeta. The Class:
Hirudinia includes leeches. These are ectoparasites.