Z LEECHES 



four or five, of which only one is dorsal to the pharynx and one is 

 on the circum-pharyngeal connectives, apparently in the process 

 of migrating dorsally. Leeches more closely resemble insects for 

 they have six pairs of ganglia in the head, usually with two placed 

 dorsally and one pair on the lateral connectives. 



The group of annelids most closely related to the leeches is 

 undoubtedly the oligochaetes. There is in fact a connecting link in 

 Acanthohdella, a leech which has a number of clear oligochaete 

 features, such as chaetae and a spacious coelom. Both leeches and 

 oligochaetes have a clitellum which secretes a cocoon for the recep- 

 tion of the eggs. Both are hermaphrodite and have well defined 

 gonads with their own ducts to the exterior. There are many 

 peculiarities of the embryology of oligochaetes which are also seen 

 in leech development. In fact, it is reasonable to regard leeches as 

 oligochaetes which have become specialized for a blood-sucking 

 ecto-parasitic mode of life. In the process they have lost the 

 chaetae which earthworms use in locomotion and have developed 

 instead a ventral sucker at each end of the body, this arrangement 

 being better adapted to clinging to the host while sucking blood. 

 Their gut has been modified for the storage of large quantities of 

 blood and with the great reduction of the coelom the segmentation 

 of the body has become obscured. 



The relationship of leeches to other annelid groups is expressed 

 in the following scheme ot classification : 



Phylum ANNELIDA 



Class POLYCHAETA Clitellum absent, chaetae borne 



on parapodia. 



Class ARCHIANNELIDA CiHated epidermis, simplified, 



possibly degenerate organiza- 

 tion. 



Class CLITELLATA Clitellum present, parapodia 



absent. 



Order OLIGOCHAETA Chaetae present, suckers absent, 



number of body segments 

 variable. 



Order HIRUDINEA Chaetae absent, suckers present, 



number of body segments 33. 



