The annelids are segmented worms that live in soil,
fresh water, or the sea. Most of the annelids are free-living, although some of
the marine forms burrow in tubes and some species (class Myzostoma) are
parasites on echinoderms. The body of an annelid is divided into a series of
similar segments and is said to be metamerically segmented. Most annelids have a
closed circulatory system where the blood is contained in vessels. Enlarged
muscular blood vessels function as hearts and pump the blood through the system
of vessels. Annelids may be dioecious (have separate sexes) or
hermaphroditic.Most annelid species go through a ciliated larval stage known as
the trochophore larva. This is a larva of evolutionary importance because the
same type
appears in several phyla.
