INTRODUCTION 6 



The adaptation of the gut of leeches to blood sucking has taken 

 place in several ways. First there is the mechanism for piercing 

 the tissues of the host. In one major group this consists of three 

 muscular ridges each shaped like half a circular saw, which can be 

 everted from the mouth and used to make a Y-shaped incision in 

 the skin. In the other group of leeches there is a very muscular 

 proboscis which is forced out of a pore in the base of the anterior 

 sucker while this is held in contact with the host. This mechanism 

 is on the whole less efficient than the former, and few leeches 

 possessing it are able to pierce the skin of a mammal. The blood 

 is prevented from clotting by the secretion of numerous uni- 

 cellular salivary glands, and is sucked into the gut by the pumping 

 action of a muscular pharynx. 



Although leeches resemble oligochaetes in being hermaphrodite, 

 they differ in having only a single male pore and a single female 

 pore. Moreover, while they have but a single pair of ovaries, they 

 normally have between ten and a hundred pairs of testes. The jawed 

 leeches transfer sperm to another leech by means of an eversible 

 penis, but those with a proboscis normally lack the penis and 

 implant a spermatophore containing sperms on the body surface of 

 another leech. After this the sperms migrate through the tissues 

 of the recipient and make their own way to the ovary. 



Leeches with a proboscis (Rhynchobdellae) have a blood system 

 of the normal annelid plan with dorsal and ventral longitudinal 

 vessels, but the jawed leeches (Gnathobdellae) have completely 

 lost their blood vessels and have instead a system of coelomic 

 sinuses in which circulates coelomic fluid containing haemoglobin. 



Before an accurate description of a leech can be given it is neces- 

 sary to determine the limits of the segments. The only external 

 evidence of segmentation in most leeches is in the arrangement of 

 the sensillae, minute whitish spots which are receptor organs for 

 tactile and light stimuli. Earlier workers adopted a convention 

 that the annulus on which these occurred should be regarded as 

 the first of its segment and this convention has been followed as 

 recently as 1941 (Bhatia) and 1945 (Miller). Castle (1900) and 

 Moore (1900) independently proposed that the nervous system 

 should be used as a basis for determining the limits of segments, 

 there being a general tendency for the parts of a segment to be 

 innervated from the ganglion of that segment and by no other 



