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The phylum Annelida comprises the segmented worms and includes the familiar earthworms and leeches, plus a great number of marine and fresh-water species that are less well known.
The most distinguishing characteristic of the phylum is the division of the body into similar segments along the antero-posterior axis. This segmentation is not restricted to the external appearance but is also reflected in the internal structures, including the reproductive organs, which are of special interest in the context of the present monograph. In the ancestral condition no well-defined gonads are present, but during the breeding season ova or spermatozoa arise from the wall of the coelum in each segment except near the anterior end. In the advanced condition the number of gonads is very much limited. Both male and female sexual organs occur in the same individual and they are permanent and often complicated structures in earthworms and leeches.
The phylum is usually divided into three classes: Polychaeta, Oligochaeta and Hirudinea. The class Polychaeta contains the marine annelids and has been generally considered to display the more primitive features of the phylum. The class Oligochaeta, which includes the fresh-water naidids and tubificids and the terrestrial earthworms and enchytraids, may have evolved from some early polychaetes. The class Hirudinea, the leeches, probably arose from some stock of fresh-water oligochaetes.