HIRUDINEA OR DISCOPHORA. 



213 



ous branches which riddle the spongy body, and have a 

 definite muscular coat. On the dorsal surface and ventrally 

 around the nerve-cord are two lacunar spaces, which are 

 really portions of the true body cavity, and not parts of the 

 vascular system. With those and similar spaces, however, 

 the blood vessels are connected by means of a secondarily 

 developed series of canals, roughly corresponding to the 

 lymphatic vessels of Vertebrates. The blood is red, and 

 contains colourless floating cells of diverse form. 



Fig. 94. — A nephridium of leech. — After Bourne. 



F., Internal terminal funnel ; C, glandular coil covered with blood 

 vessels ; V., external terminal vesicle. 



Excretory system. — There are seventeen pairs of 

 excretory tubules or nephridia, from the second to the 

 seventeenth segment inclusive. These open laterally on the 

 ventral surface, voiding the waste products extracted from the 

 blood vessels which cover their walls. From the seventh to 

 the seventeenth, each nephridium ends internally in a 

 ciliated " cauliflower " lobe, corresponding to the funnel of 

 Oligochaeta, and enclosed in a blood space, apparently part 

 of the reduced ccelom. In the first nine of these funnel- 



