128 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



cord of the same side, and probably with one of the lon- 

 gitudinal fibers which form part of the dorsal nervous 

 system. We thus have two nerves entering the disc on 

 each side of the median line, each of which is the com- 

 bined product of two ventral and one dorsal cord. 



Upon entering the disc each nerve passes outward a short 

 distance (fig. 2), and divides into two branches, one ex- 

 tending forward, the other backward. The former, after 

 giving rise to several small nerves which supply the antero- 

 lateral portions of the disc, unites with its fellow of the 

 opposite side, forming an anterior, semicircular commis- 

 sure. From this, four well defined nerves pass forward, 

 and branching freely, supply the anterior portion of the 

 disc, including its marginal membrane. The nerves pass- 

 ing backward are united by a commissure which passes 

 through the center of the disc. Beyond this point each 

 breaks up into a series of fibers which penetrate through- 

 out the posterior half of the sucker. The finer branches 

 of each nerve form an anastomosing network of fibers 

 which appear to unite with similar anastomoses which are 

 the product of large branches in other parts of the disc. 



Returning to the four ventral longitudinal cords, we find 

 them constituting a part of an extensive system which in 

 some respects resembles that of Tristomtim tnolcB, as deter- 

 mined by Lang (1881). Between the inner pair are from 

 eighteen to twenty-four commissures, of which the first one 

 is excessively delicate, and consists of a very few nerves. 

 It is perfectly constant, however, and lying against the phar- 

 ynx may be traced to each side to a point a short distance 

 posterior to the first commissure between the inner and outer 

 longitudinal nerves. The succeeding connectives, which 

 cross the reproductive organs, are also delicate, but they 

 usually stain readily with methylen blue, and are then easily 

 followed. Behind the testes the cross nerves become stronger 

 and are demonstrable in sections. The nerves uniting the 

 inner and outer cords are larger, and in many cases arise 

 opposite the roots of the inner commissures. Unlike the 

 latter, the most anterior one is very strongly developed, 

 while the succeeding ones are of smaller caliber. 



