54 THE LEECH 



epidermis and of the nephridia should be specially 

 noticed. 



The capillaries are directly continuous in various 

 parts of the body with brown pigmented fibres, 

 similar to those of the dermis already described. 

 5. The botryoidal tissue consists of a network of vessels, 

 the channels of which are irregular in size and often 

 end bUndly, and the walls of which are formed of 

 large granular pigmented cells which project as 

 irregular rounded swellings. 



These curiously swollen vessels occur in great 

 abundance around the viscera, immediately within 

 the longitudinal muscle-layer. They are in free com- 

 munication with the ordinary blood-capillaries. 



F. The Excretory Organs. 



The nephridia have already been fully described. The 

 details of their structure, and more especially the intracellular 

 network of ductules, and the intercellular capillary plexuses, 

 can be very readily made out in transverse sections, and 

 should be carefully studied. The structural difi'erences in 

 various parts of the organ should be noted, and particularly 

 the head of the testis-lobe, which lies in the peri-nephrostomial 

 sinus. 



G. The Reproductive Organs. 



1. The testes are seen, if the section happens to pass 



through them, as spherical sacs lying at the sides of 

 the ventral sinus, and containing groups of sperma- 

 tozoa in various stages of development. 



2. The vasa deferentia lie to the outer sides of the testes : 



each is a thick-walled tube, with a small central 

 lumen. 



H. The Nervous System. 



The nerve-chain lies in the ventral sinus, and is readily 

 recognised. Its double character is very obvious in transverse 

 section, and if the section passes through a ganglion the large 

 pyriform nerve-cells can be well seen. 



