32 WRIGHT AND MACALLUM. [Vol. I. 



is followed from the cell-body. The nerve-fibres acquire no 

 color by this method of treatment; each fibre appears to be a 

 simple tube filled with a clear homogeneous jelly-like fluid, to 

 which only gold chloride gives a color. The crossing and inter- 

 twining of these fibres in a nerve-cord give the appearance 

 usually known as "spongy," — an appearance quite common in 

 Trematodes and Cestodes. As to the internal structure of these 

 fibres, beyond what is said, we have determined nothing. 



The Intestinal Canal. 



Reference has been made above to the extremely mobile 

 character of the upper and lower lips, and to their acting as an 

 oral sucker. The mouth leads into a prepharynx, such as has 

 now been observed in so many Trematodes, and in this cavity 

 the muscular pharynx has free play, alternately opening to swal- 

 low the contents of prepharynx, and closing to discharge these 

 backwards into the intestine. Neither the prepharynx nor 

 muscular pharynx call for special remark. The histology 

 of the latter appears to be very similar to that in other 

 Trematodes. What we have to say specially as to the large 

 cells present between the contractile fibres will be found 

 above. 



There is no oesophagus, the muscular pharynx opening 

 directly into the anterior arch of the intestine. The lateral 

 intestinal canals are simple without cceca, and they terminate 

 by the formation of a simple posterior arch. It will be re- 

 membered that a similar arch is present in Polystomum 

 integerrimuin, complicated, however, by giving off numerous 

 coeca into the caudal lamina, whereas in P. ocellatiim and 

 P. oblongiim the intestinal branches end coecally without such 

 an anastomosis. 



The wall of the intestinal branches is composed of a muscular 

 and an epithelial layer, the thickness of the latter varying 

 widely, while that of the former is never greater than 2.$ fi. 



The muscular fibres are circular, longitudinal ones having 

 been observed only anteriorly. The circular fibres have 

 elongated oval nuclei, measuring 20 ^u X 3.5 [.i which curve 

 with the fibres on the epithelial layer. Capillaries of the 

 excretory system appear frequently to pierce the muscular 

 layer. 



