Anatomy and Physiology
Like other annelids, the leech is a segmented animal. But unlike other annelids, there is no correspondence between the external segmentation of a leech's body surface with the segmentation of its internal organs. The body surface of the animal can be divided into 102 annuli, whereas its internal structures are divided into 32 segments. Of the 32 segments within the body, the first four anterior segments are designated as head segments, which include an anterior brain and sucker. This is followed by 21 midbody segments, which include 21 neuronal ganglia, two reproductive organs, and 9 pairs of testes. Finally, the last seven segments are fused to form the animal's tail sucker, as well as its posterior brain. The leech also has 32 brains because a brain segment is located in each segmentation of the body.
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