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The leech is an invertebrate animal belonging to the phylum Annelida, a zoological category that includes more than 15,000 species of segmented bristle worms and 650 species of leeches in the subclass Hirudinea.

I. Nephridia.—Excretory organs which are undisputed nephridia are practically universal among the Oligochaeta, Hirudinea and Archiannelida, and occur in many Polychaeta.

To this category belong certain sacs and pouches in many, perhaps most, genera of the Oligochaeta family, Eudrilidae, and possibly the gonad ducts in the Hirudinea.

Gonads and Gonad Ducts.—The gonads and their ducts in the Hirudinea invariably form a closed system of cavities entirely shut off from the coelom in which they lie.

Hirudinea.—The leeches are more particularly to be compared with the Oligochaeta, and the following definition embraces the main features in which they agree and disagree with that group.