Shape and size: The body of a leech is soft, vermiform, elongated and segmented. It becomes ribbon shaped when extended and almost cylindrical when contracted. Leeches may grow to a length of 35cm.
Colouration: Dorsal surface is olive green in colour and the ventral surface is orange yellow or orange red in colour.
Segmentation: Metamerism is the segmentation of the body. The body of leech is metamerically divided into 33 segments. The segments are arranged one behind the other. Each segment is further superficially subdivided into rings or annuli. A temporary clitellum is formed on segments 9-11, which is meant to produce a cocoon during the breeding season.
Receptors: On the dorsal side there are five pairs of eyes on the first five segments. Each segment bears a number of sensory projections called receptors are located in each annulus and segmental receptors are located on the first annulus of each segment.
Suckers: leech has two suckers. The sucker located at the anterior end is called anterior sucker or oral sucker which is ventral in position occupying the first five segments. The posterior sucker is formed by the fusion of the last seven segments. The anterior sucker helps, in feeding, while both suckers help in attachment and locomotion.
External apertures
Mouth: It is located in the middle of anterior sucker.
Anus: Anus is a small aperture that opens on the mid- dorsal side of 26th segment.
Nephridiopores: Nephridia open to the exterior by 17 pairs of nephridiopores. They lie ventrally on the last annulus of each segment from 6 to 22.
Male genital pore: It is a mid – ventral opening, situated between second and third annuli of 10th segment.
Female genital pore: It lies mid- ventrally between second and third annuli of 11th segment.
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