IV HIRUDINEA 155 



slender pear-shaped or club-shaped cells of great size. These are uni- 

 cellular glands which discharge their secretion into the cavity of the 

 pharynx, so that it mixes with the blood as it is being ingested. This 

 secretion has the remarkable property of preventing the coagulation of 

 blood, and owing to its presence the blood taken into the crop remains 

 perfectly fluid and unclotted for weeks or even months, so that it can 

 slowly pass onwards into the intestine to undergo the process of digestion. 

 The actual digestive ferment is apparently secreted by the lining cells 

 of the two rounded intestinal caeca {d.g). 



A characteristic feature of Hirudo, as of the majority of leeches, 

 lies in the fact that the coelome is to a great extent obliterated, the 

 open coelomic body-cavity seen in other annelids being replaced by a 

 dense spongework. Here and there the cavity persists in the form of 

 fluid-containing spaces or sinuses, one of which encloses the nerve- 

 cord ventrally while another runs longitudinally in a dorsal position. 

 Round the limit of the coelome, immediately underlying the body-wall, 

 is a network of small tubular cavities characterized by the dark pigment 

 in the tissue surrounding them — the botryoidal tissue. 



A remarkable peculiarity of the coelomic fluid of Hirudo is that it 

 is coloured red by the presence, in solution, of haemoglobin — the red 

 iron-containing pigment which is present in the blood of various animals. 

 It would appear from this that the coelomic fluid has in the leech taken 

 over functions normally pertaining to the blood. The spongework of 

 coelomic spaces with their contained coelomic fluid seem to have replaced 

 the blood system functionally and in correlation with this the original 

 blood-system seems to have to a great extent disappeared. Along 

 each side of the body of the leech there runs a conspicuous lateral 

 " blood-vessel " with contractile muscular walls but this communicates 

 freely with the coelomic spaces and is filled with coelomic fluid and the 

 probability seems to be that even it is not a true blood-vessel but merely 

 a coelomic sinus with specially muscular wall. 



The leech possesses normally seventeen pairs of nephridial tubes, the 

 internal funnel of which is broken up into a spongework of fine pores and 

 is contained in a special little coelomic cavity. Just before perforating 

 the body-wall the nephridium dilates to form a bladder, spherical in shape 

 when distended. The opening to the exterior is a minute pore situated 

 laterally and on the ventral side of the leech. It can most easily be 

 detected by the drop of fluid which oozes from it when the surface of 

 the leech is carefully dried and the animal subjected to slight pressure. 

 An important point to notice is that the nephridial openings over the 

 greater part of the body occur on each fifth annulus — an indication of 



