MEDICINAL LEECH. 241 



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. 



Excretory system. There are seventeen pairs 01 

 excretory tubules or nephridia, from the second to the 

 eighteenth segment inclusive. These open laterally on the 



FIG. 125. A nephridium of leech. After Bourne. 



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

 vessels ; V., external terminal vesicle. 



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-bearing nephridia the terminal lobe lies close upon 

 and dorsal to a testis, but there is no morphological 

 meaning in this approximation. Each consists of two 



