196 ANNULATA. 
complex system of fine vessels called the botryotdal tissue. 
Through the medium of this tissue the blood-vascular and 
ccelomic 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 subcesophageal 
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 
leech not only progresses by its suckers, but 
it can swim rapidly by undulating motion 
of the whole body. (See Fig. 116.) ; 
There are seventeen pairs of nephridia found in segments 
6 to 22 inclusive. A typical nephridum has (1) an internal 
branched but c¢/osed funnel which rests in a 
small cavity of the ccelom, (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 zestes 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 ro and then unite and pass down the 
penis; at the base of the penis is a small Jrvostate gland. 
The female organs are in segment 11 and are formed by 
paired ovdsacs, 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. 
Nervous. 
Muscular. 
Excretory. 
