No. 2.] ANATOMY OF BDELLODRILUS ILLUMINATUS. 527 



Excretory system. — The number of nephridia is reduced, as 

 in Branchiobdella (Dorner, Vejdovsky, etc.), to two pairs. Of 

 these, one pair is located symmetrically entirely within the 

 eighth somite (may communicate with the seventh), opening 

 to the exterior by small non-pulsating vesicles on the dorsal 

 surface of the body between the eighth and ninth somites. In 

 structure they are similar to the anterior ones (Fig. 34). 



The anterior pair alternate in position, one occupying the 

 body cavity to the left of the alimentary canal in the second 

 and third somites, and extending forward to the right side of 

 the first ; the other lying mostly within the fourth somite, of 

 which its most conspicuous portion occupies the right side. 

 The cellular masses into which the nephridial tubules plunge 

 (compare Dorner) are darkly colored and conspicuous in the 

 living animal, that belonging to the anterior nephridium being 

 located on the left side of the third somite, that of the more 

 posterior to the animal's right in the fourth somite. 



The somewhat complicated course of the tubules has not 

 been fully unravelled in detail, so that the present account will 

 be confined to the statement of a few clearly ascertained facts. 

 The anterior pair of nephridia have a common external opening 

 through a median vesicle on the dorsum of the third major 

 annulus. This is formed from an invagination of the epi- 

 dermis, which lines it, and a covering of muscle fibres (Fig. 33). 

 An internal proliferation of its lining cells forms a nearly com- 

 plete valve-like diaphragm, which divides the interior into an 

 anterior larger compartment opening to the exterior by a pore, 

 and a posterior smaller one which receives the terminal por- 

 tions of the nephridial tubules, and communicates with the 

 anterior through an opening in the diaphragm. The terminal 

 portions of the tubules, which are of large size, pass from the 

 vesicle transversely around the body, one to the right, the 

 other to the left, between the two muscular layers ; and, enter- 

 ing the body cavity close to its floor, diminish decidedly in 

 diameter (Fig. 35), and then pass into a long tubule which is 

 arranged in a complicated series of long loops, which pass over 

 and around the intestine, and, in the case of the left nephrid- 

 ium, reach far forward into the first somite, and, of the right 



