Annelida: Leech Anatomy

Leeches are the only annelids with a fixed number (34) of body segments; each segment has secondary subdivisions known as annuli. A clitellum, less conspicuous than that of oligochaetes, is present; there are no parapodia. A leech has a small anterior sucker and a larger posterior one; the leech crawls by moving the anterior sucker forward, attaching it, and drawing up the posterior sucker. Most leeches can swim by rapid undulations of the body, using well-developed muscles of the body wall.

The coelom differs from that of other annelids in that it is largely filled in with tissue. Coelomic fluid is contained in a system of sinuses, which in some leeches functions as a circulatory system; there is a tendency in this group toward the loss of true blood vessels. The blood of some leeches is red. In others the blood lacks oxygen-carrying pigments and is therefore colorless; the oxygen dissolved directly in the blood is sufficient for respiration. Gas exchange occurs through the body surface of most leeches, although many fish-parasitizing leeches have gills.

The sense organs consist of sensory cells of various types, including photoreceptor cells, scattered over the body surface. There are also from 2 to 10 eyes, consisting of clusters of photoreceptor cells and located toward the front of the body.

Sections in this article:

• Introduction
• Leech Reproduction
• Leech Predation and Digestion
• Leech Anatomy
• Class Hirudinea
• Oligochaete Reproduction
• Oligochaete Circulation and Respiration
• Oligochaete Digestion
• Oligochaete Anatomy
• Class Oligochaeta
• Archiannelida and Myzostomaria
• Polychaete Reproduction
• Polychaete Anatomy
• Sedentary Polychaetes
• Errant Polychaetes
• Class Polychaeta
• Reproduction
• The Nervous System
• Respiration
• Circulation
• Digestion
• The Body Wall
• Segmented Bodies
• Bibliography

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