BRANCHES — BRANCHIAL NERVES OF FISHES 149 



Polyodon is said by Jordan ('05) to be a bottom feeder of 

 sluggish habits, and its long snout is said to be more or less 

 sensitive and to be used to stir up the mud in which are found 

 the minute organisms on which the fish feeds. The fish abounds 

 in the Mississippi, which is always a markedl}^ muddy river, and 

 Psephurus, a closely allied species, is found in the Yangtse and 

 Hoang Ho, which are also muddy rivers. Vision can therefore 

 be of but relatively Httle value to these fishes, and, in accord 

 with this, the eyes of Polyodon are small. This lack of visual 

 power should then be compensated for by some other sense, if 

 the fish is not to be too severely handicapped in its search for 

 food, and the nerve pits seem to have been developed for this 

 purpose. What their function is, can, of course, only be deter- 

 mined by experiment, but response to pressure would here 

 seem to be of little, if any, value, while a keen sense of either 

 touch or taste would be of great advantage; but if they respond 

 to either of the latter stimuli, the central origin of the fibers that 

 innervate them would not correspond to that of the fibers that 

 innervate the organs of corresponding sense in the Teleostei. 

 In the numerous catfishes that inhabit the Mississippi and have 

 habits and visual powers similar to those of Polyodon, a keen 

 sense of taste has been developed in numerous terminal buds on 

 the external surface of the head (Herrick, '03), but none of these 

 buds could be found anywhere on the external surface of the 

 head of my 140-mm. specimens of Polyodon excepting only 

 along the upper and lower margins of the mouth, and but few 

 of them even there. As a few nerve pits (small pit organs) are 

 found on the outer surface of the head of the catfishes, it may be 

 that the conditions there represent the last stages of the dis- 

 placement of one sense organ by another responding to the 

 same stimulus but better adapted to the purpose. 



The nervus trigeminus arises by two roots, one motor and 

 the other general sensory, the latter root being connected with 

 the root of the nervus profundus by an intracranial bundle of 

 fibers. The motor and general sensory roots run outward ven- 

 tral to, and wholly independent of, the anterior portion of the 

 intracranial lateralis-communis ganglion and issue from the 



