About 600 species of leeches are known. The cattle Leech, found in ponds, lakes and swampy areas of India is a common species that sucks blood from various vertebrates, particularly cattle that frequent water. Almost 600 species of leeches are known. Hirudo medicinalis has been used to treat patients for centuries for treating abscesses, painful joints, glaucoma, myasthenia gravis and thrombosis.
Leech is vermiform, bilaterally symmetrical and metamerically segmented animal that has a broad posterior sucker for attaching to substratum. It is olive-green in colour and sometimes with stripes of lighter colour and can reach a length of 30 cm. Body is metamerically segmented into 33 segments or somite, each of which is further subdivided into annuli by grooves. A temporary clitellum or cingulum is formed on segments 9-11, which is meant to produce a cocoon in breeding season. The anterior sucker helps in attachment as well as feeding. On the dorsal side there are 5 pairs of eyes on the first five segments. Mouth is in the middle of anterior sucker and anus is a small aperture that opens on the dorsal side of 26th segment.
Nephridia open to the outside by 17 pairs of nephridiopores on segments number 6 to 22. Male genital opening is in the middle of 10th segment and female genital pore exists midventrally on 11th segment.
Skin contains many types of glands, namely, Slime glands, which are modified mucous glands that are distributed all over the body surface. Sucker glands are situated on anterior and posterior suckers and provide them with adhesive properties. Prostomial glands are present on prostomium and their secretion forms plugs on the anterior and posterior ends of cocoon. Clitellar glands are present on segments 9-11and secrete a frothy girdle that dries to form a cocoon inside which eggs are deposited.
Mouth is a tri-radiate aperture situated in the middle of the anterior sucker. There are three jaws, one is dorso-median and the other two are ventro-lateral. Jaws are provided with papillae which bear the openings of salivary glands. Pharynx extends from 5th to 8th segments, on the outer side of which are unicellular salivary glands that secrete hirudin, which prevents coagulation of host blood during feeding. Radial muscles dilate the pharynx and carry out suction of blood. The crop is the largest chamber of alimentary canal and extends from 9th to 18th segments, one chamber in each segment and a total of ten chambers. A pair of caeca project out laterally from each chamber, their length increases towards the posterior side and the last pair of caeca extends as far as 22nd segment. The crop leads to stomach whose walls are produced internally into transverse folds. The next chamber is intestine which is a small straight tube located in 20-22nd segments and narrows down at the posterior end into rectum, which opens on the dorsal side of 26th segment by anus. The sucked blood is stored in crop and enters for digestion in stomach and intestine drop by drop and takes lot of time for digestion.
There are no special respiratory organs in leech. Skin also serves respiratory function as epidermis is a permeable membrane through which the carbon dioxide and oxygen dissolved in water can be exchanged. The skin is always kept moist by the mucus secreted by the slimeglands which prevents it from drying even on land.
The blood vascular system is represented by haemocoelomic channels, which are filled with blood-like fluid that contains haemoglobin dissolved in plasma and colourless corpuscles. The haemocoelomic system consists of four longitudinal channels, two of which run ventro-laterally, one on each side, one runs along the mid-dorsal side and the other along the mid-ventral side. The four channels are connected with each other in the posterior as well as anterior region of body. Branches from these channels supply blood to different organs.
The dorsal haemocoelomic channel supplies blood to dorsal and dorso-lateral parts of the body and the alimentary canal. The blood from these parts is collected by the lateral channels.
The ventral haemocoelomic channel supplies blood to the ventro-lateral body walls and nephridia, from where the blood returns to the lateral channels. Lateral channels distribute blood to the nephridia, genital organs, gut and ventral body wall.
Like other annelids excretory system consists of nephridia, which are 17 pairs of small coiled tubules embedded in syncytium and arranged segmentally, one pair in each segment from 6th to 22nd segment. They are of two types, Testicular nephridia and Pre-testicular nephridia. Testicular nephridia are 11 pairs, located from 12th to 22nd segment and their initial lobes end in the testis sacs in each of these segments. Pretesticular nephridia are present in segments 6-11 and their initial lobe ends blindly in the botyoidal tissue as there are no testis sacs in these segments.
Each nephridium consists of 4 lobes, vesicle and ciliated organ. Ciliated organ is enclosed within the perinephrostomal ampulla and manufactures coelomic corpuscles. The initial lobe is a long, tube-like structure that twines around the apical lobe. Apical lobe lies in the antero-posterior position beneath the gut. Main lobe is horse-shoe shaped and lies between the adjacent diverticula of the crop behind the apical lobe. The inner lobe is narrow and lies in the inner concavity of the main lobe and connects forward with the apical lobe. Vesicular duct emerges from the anterior end of main lobe and passes over and joins a large oval bag called vesicle or bladder. From the vesicle emerges a short excretory duct which opens to the exterior by a nephridiopore.
The ciliated organ has no excretory function in adults but manufactures coelomic corpuscles. The nephridia serve to eliminate excess water and nitrogenous wastes. Nitrogenous waste consists mainly of ammonia and small quantities of urea. Nephridia also serve as osmoregulatory organs. The nephridium is richly supplied with branches of haemocoelomic channels. Its gland cells separate the waste products from the haemocoelomic fluid and transport it to the vesicle, from where they are discharged through the nephridiopore. Some scientists also assign excretory function to the botryoidal tissue because the capillaries in it are in communication with the haemocoelomic fluid.