(1) The leech swims gracefully through water by undulating its body in a snake-like fashion. The botryoidal tissue is believed to remove waste products but a definite proof in favour of this assumption is wanting. Several canals are enclosed in a thin sheath of connective tissue which adheres to the gut internally and longitudinal muscles externally. The free ends of the canals communicate with the hoemocoelomic channels and they are filled with hoemocoelomic fluid. It consists of a network of minute canals which are loaded with a dark brown pigment. The botryoidal tissue lies just beneath the longitudinal layer of muscles. These are known as the hoemocoelomic channels. The space is obliterated by the botryoidal tissue and reduced to certain longitudinal channels filled with blood. In the adult leech there is no distinct body cavity. The coelom or body cavity is distinct in a leech embryo. With the help of this, the layer of longitudinal muscles is firmly adherent to the wall of the underlying alimentary canal. The musculature encloses the body cavity which is filled up by a peculiar kind of spongy connective tissue called botryoidal tissue. The muscles are composed of highly elastic spindle-shaped cells which help the leech to carry on locomotion. There is an outer layer of circular muscle fibres running round the body, and an inner layer of longitudinal muscle fibres extending along the length of the body.īesides these, there are other bundles running obliquely and from dorsal to ventral surface. This constitutes the thickest part of the body wall. The cuticle, epidermis and dermis together form the skin of the leech. It consists of fibres, capillary loops and brightly coloured pigment cells, which extend into the epidermis. (3) The dermis is a thin basement membrane beneath the epidermis. Some of the epidermal cells are converted into pear-shaped glands which secrete mucus for moistening the skin others are modified to form the various receptors which act as sense organs. The epidermis, therefore, forms a vascular membrane through which exchange of gases during respiration may readily take place. The spaces between the narrow inner ends of the epidermal cells are filled with a network of capillaries, a small amount of connective tissue and a large number of pigment cells, all of which are derived from the underlying dermis. (2) The epidermis is a single layer of columnar cells with their broad ends directed outwards. ![]() It is secreted by the underlying epidermis and is cast off periodically to be replaced by a new one. (1) The cuticle is a transparent non-cellular membrane which forms a delicate protective covering for the entire body. (2) The anus is a small opening situated in the middle line on the dorsal surface of the 26th segment, in front of the posterior sucker. (1) The mouth is a triradiate opening situated ventrally in the centre of the cup-shaped anterior sucker. It is used as an organ for adhesion and locomotion. It is a muscular disc, circular in Outline, and is larger in size than the anterior sucker. The posterior sucker is composed of the last seven segments. The anterior border of the sucker forms an upper lip which is highly sensory and bears the receptors for touch as well as taste. The ventral hollow of the anterior sucker leads into the mouth. There are five pairs of prominent segmental receptors on the dorsal surface of the anterior sucker. It is oval and cup-shaped, with its hollow towards the ventral surface. The anterior sucker is composed of the first five segments. There is an adhesive sucker at either end of the leech. ![]() It will be found that at both ends the body, a segment may be composed of only one or two rings. The limits of the true segments are determined by the position, and a number of the segmental receptor organs and not on the basis of the superficial rings. The receptors are the sense organs of the leech and they appear as small black spots on the surface of the skin.Įach metamere bears four pairs of segmental receptors on its dorsal surface and three pairs on the ventral surface. These external rings, however, are only superficial structures and do not represent the actual number of true segments or somites in the body, the number of which is only 33.Ī typical segment or metamere from the middle of the series includes five rings, of which the first bears the segmental receptor organs arranged in a ring round the body. The entire body of a leech is divided externally into a large number of rings or annuli separated by transverse grooves. The two flanks are striped with black, yellow and orange bands. The dorsal surface is dark-green and the ventral surface is pale yellow in colour.
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