472 W. s. NICKERSON, 



shall have been found and its anatom) studied. The fact that it is 

 absent in Aspidogaster and Cotylogaster , which are evidently very 

 closely related forms, makes it improbable that it \vill be found to 

 occur in Stichocotijle. 



Nervous System. 



The nervous system conforms closely to the typical condition in 

 Trematodes. A diagram of it is given in PI. 30, Fig. 12. A broad 

 commissure composed ot fine fibrils, closely interwoven extends across 

 above and just in front of the pharynx (PI. 29, Fig. 5). Upon each 

 side of the median plane this enlarges a little, and from the two 

 enlargements thus formed arise three pairs of nerves or fibre-courses. 

 Those forming the first pair are short and extend directly forward 

 above and upon either side of the pre-pharyux to about midway 

 between the commissure and the anterior tip of the body, where they 

 become dispersed and cannot be followed farther. Those of the second 

 pair are very small and extend downward and backward to the vicinity 

 of the pharynx. The third pair are the most important and make 

 up the greater part of the nervous system. They extend diagonally 

 downward and backward upon each side of the pharynx to the ventral 

 region and thence backward toward the posterior end of the body 

 just outside the margins of the ventral suckers. The position of these 

 ventral nerves as seen in cross section is shown in PI. 29, Fig. 4 cd. n. v. 

 Opposite each sucker a pair of minute branches is given off, one from 

 each lateral nerve; these extend inward to the sucker. There is no 

 sub oesophageal commissure, and no direct connection between the 

 nervous structures of the two lateral halves of the body other than 

 that of the supra-oesophageal commissure already described. 



The histological condition of this system is difficult to make out 

 satisfactorily. None of the tracts of fibres described contain numerous 

 ganglionic cells or nuclei. In the large ventral nerves a few scattered 

 nuclei are present, but I have not observed such in anj of the other 

 nerve courses. There are, however, abundant nuclei lying immediately 

 upon the surface of the fibre tracts in nearly all regions. Along the 

 course of the two lateral nerve cords these seem to be most abundant 

 upon the ventral side. In form and size, as well as in staining 

 qualities, they do not difier noticeably from the other nuclei distri- 

 buted through the parenchyma. Notwithstanding this fact, occasionally 

 one may be found which clearly has a fibre passing off from one of 

 its ends and extending into the tract of nerve fibres. Now and then 



