ZooL— Vol. III.] HEATH— EPIDELLA SQUAMULA. 127 



of the body. The remaining pairs of nerve-bundles join the 

 semicircular commissure, and many of their fibers may be 

 traced into some of the several nerves which pass on into 

 the substance of the sucking discs. Posteriorly the com- 

 missure bends inward on each side, and unites with the 

 brain at the origin of the outer longitudinal cords. 



As Lang noted in Tristomum moleB, the brain is not only 

 symmetrical in form, but the constituent cells of one side find 

 their counterpart in the other. In B. squamiila, this is not 

 only true of the brain, but of the nerves and semicircular 

 commissure above described. Throughout these cords 

 bipolar cells are of frequent occurrence, and generally one 

 nerve from each may be traced close to the surface of the 

 body, while the proximal fiber passes into the substance of 

 the brain. In a few cases the proximal nerves, instead of 

 traversing the brain of the same side, cross over and de- 

 cussate with the fibers from corresponding cells of the 

 opposite side. Another interesting arrangement is found 

 in cells of a tripolar type, two of which are shown in fig. 5. 

 Here one nerve leads to the brain; another passes into the 

 substance of the sucker on the same side; while the remain- 

 ing fiber passes across to innervate the sucker of the 

 opposite side. Other less frequent or more complicated 

 types may perhaps be described in a future paper. 



As noted above, the inner and outer pairs of great longi- 

 tudinal nerves arise from the lateral borders of the brain, 

 and extend through the body, to break up into a complex 

 system of nerves within the posterior sucker. In passing 

 through the pedicel they are of necessity brought close 

 together, but the paths which they pursue in reaching this 

 point are considerably different. The inner pair, the 

 stronger of the two, in passing backward diverges slightly 

 to skirt the ovary, and more widely to clear the testes. 

 From this point onward they gradually approach each 

 other, and enter the sucking disc close together. Each 

 cord of the outer pair directs its course toward the lateral 

 margins of the body, which it follows to the posterior end 

 of the animal. Here it unites with the inner longitudinal 



