474 W- s NICKERSON, 



The iuner end 1 have been able to trace with certainty to a 

 gaDglionic cell in only a single case (PI. 30, Fig. 14). In a few other 

 cases, iiowever, I have succeeded in following it back to the immediate 

 vicinity of the ganglionic cells which thickly border the first pair of 

 nerve bands. I have also found stumps of similar fine fibres con- 

 necting with certain of these cells. Within the suckers I have in a 

 few cases found cells terminated by processes at two opposite poles. 

 These facts furnish what seems to me satisfactory evidence that the 

 structures in question are of a sensory character. Their distribution 

 also is such as to lend support to this view. They are present at 

 the anterior tip of the body, and all around the margin of the oral 

 sucker and within it. They are also found scattered over the anterior 

 ventral part of the body, between the mouth and the first sucker, the 

 region in which the sexual orifice is located. Upon the anterior 

 suckers they are very numerous, becoming less so in those farther 

 back toward the caudal end. The number in each sucker was counted 

 in the case of one worm, which was sectioned in such a way as to 

 be very favorable for seeing these structures. The result is as follows: 

 \Mthin oral sucker (about) 75 



19th fS 



11 A-^ „ „ „ O 



It will be seen that there are very few of them in the younger 

 suckers of the posterior part of the row. Nevertheless I have seen 

 occasionally one of these sense organs upon the ventral side very 

 near the caudal end, and outside of the suckers. I have never seen 

 one upon the dorsal side of the body except as already stated at the 

 anterior tip. 



In PI. 30, Fig. 15 I have represented a condition which I have 

 observed in a few cases , and which I believe to be a sense organ 

 in process of formation. Careiul examination of both this and the 



