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 ot 
excretory tubules or nephridia, from the second to the 
eighteenth segment inclusive. These open laterally on the 
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Fic, 125.—A nephridium of leech.—After Bourne. 
#., 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 Oligocheeta, 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 
16 
