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Anatomy and physiology

Leeches show a remarkable similarity to each other in morphology, very different from typical annelids which are cylindrical with a fluid-filled space, the coelom (body cavity). In leeches, most of the coelom is filled with botryoidal tissue, a loose connective tissue composed of clusters of cells of mesodermal origin. The remaining body cavity has been reduced to four slender longitudinal channels. Typically, the body is dorso-ventrally flattened and tapers at both ends. Longitudinal and circular muscles in the body wall are supplemented by diagonal muscles, giving the leech the ability to adopt a large range of body shapes and show great flexibility. Most leeches have a sucker at both the anterior (front) and posterior (back) ends, but some primitive leeches have a single sucker at the back.

Like most annelids, with a few exceptions like Sipuncula, Echiura and Diurodrilus, the leech is a segmented animal, but unlike other annelids, the segmentation is masked by secondary external ring markings (annuli). The number of annulations varies, both between different regions of the body and between species. In one species, the body surface is divided into 102 annuli. All leech species, however, have 32 segments, called somites, (34 if two head segments, which have different organization, are counted). Of these segments, the first five are designated as the head and include the anterior brain, several ocelli (eyespots) dorsally and the sucker ventrally. The following 21 mid-body segments each contain a nerve ganglion, and between them contain two reproductive organs, a single female gonopore and nine pairs of testes. The last seven segments contain the posterior brain and are fused to form the animal's tail sucker. The septa that separates the body segments—and the mesenteries which in turn separates each segment into a left and right half—in the majority of annelids, have been lost in leeches except for the primitive genus Acanthobdella, which still have some septa and mesenteries.

The body wall consists of a cuticle, an epidermis and a thick layer of fibrous connective tissue in which are embedded the circular muscles, the diagonal muscles and the powerful longitudinal muscles. There are also dorso-ventral muscles. In leeches the original blood vascular system has been lost and replaced by the modified coelom known as the haemocoelomic system, and the coelomic fluid, called the haemocoelomic fluid, has taken over the role as blood. The haemocoelomic channels run the full length of the body, the two main ones being on either side. Part of the lining epithelium consists of chloragogen cells which are used for the storage of nutrients and in excretion. There are 10 to 17 pairs of metanephridia (excretory organs) in the mid-region of the leech. From these, ducts typically lead to a urinary bladder, which empties to the outside at a nephridiopore.