Taxonomy

Abstract

The Oligochaeta are related to the Polychaeta (bristle worms) and the Hirudinea (leeches). Polychaeta and Hirudinea are exclusively marine whereas oligochaetes (except for a few species secondarily adapted) inhabit either soil or fresh water. Polychaetes may be considered as the older group because their larval development is more primitive than the oligochaetes, which have an embryo in a cocoon supplied by yolk or albumen. However, the greatest differences between the orders are in the structure of their genital organs. Thus the genitalia of the Polychaeta, which have separate sexes, are simple, with extensive production of sexual cells from the coelomic epithelium, and sexual products that are expelled into the sea by rupturing the body wall. The oligochaetes, by contrast, are hermaphrodite, with their sexual organs confined to two or three segments and a very specialized and complex mechanism of fertilization and dispersal of eggs.