196 ANNULATA. 



complex system of fine vessels called the boiryoidal tissue. 

 Through the medium of this tissue the blood-vascular and 

 coelomic systems are said to communicate. 



The nervous system is on the annelid plan, but somewhat 

 concentrated. A double brain lies on the pharynx, from 

 which there passes a pair of commissures round 

 the pharynx, meeting below to form the double 

 ventral nerve-cord along the body. On this cord there 

 are twenty-three ganglia which are segmentally arranged. 

 The first is larger than the rest and forms a suboesophageal 

 mass consisting probably of five fused ganglia. The last, 

 supplying the sucker, is said to contain seven fused ganglia., 

 the sucker itself being supposed to represent seven fused 

 segments. 



The muscles consist of powerful circular and longitudinal 



series and a number of dorso-ventral muscles as well. The 



ular ^^^^ "^^ ^^^y progresses by its suckers, but 



it can swim rapidly by undulating motion 



of the whole body. (See Fig. ii6.) 



There are seventeen pairs of nephridia found in segments 

 6 to 2 2 inclusive. A typical nephridum has (i) an internal 

 branched but closed funnel which rests in a 

 small cavity of the coelom, (2) a much-coiled 

 excretory portion with a ciliated duct, and (3) a bladder or 

 vesicle opening to the exterior by the nephridiopore. The 

 first four nephridia are without -the funnel. 



The leech is hermaphrodite and the male organs are 

 segmented. They consist of nine pairs of testes in segments 

 12 to 20 inclusive. Each opens inwards by a vas efferens 

 into a vas deferens on each side. These are coiled into 

 epididymes in segment 10 and then unite and pass down the 

 penis ; at the base of the penis is a small prostate gland. 



The female organs are in segment 11 and are formed by 

 paired ovisacs^ containing in their cavities the ovaries., which 

 unite to form an oviduct swollen at its distal end into a 

 vagina and opening to the exterior on the same segment. 



Fertilisation is mutual and the eggs are laid in a cocoon, 

 usually in damp places. They have yolk and pursue an 

 embryonic development. 



