360 W. J. CROZIER 



move, as Parker ('11) has described for C. fornicata, by means of 

 direct waves. In one of these crepidulas the use of the whole 

 foot as a sucker was very clear, and in fact sometimes during 

 creeping the circumference only of the foot was in contact with 

 the substratum (of smooth glass). 



Ischnochiton was of particular interest, as it moved in a pos- 

 terior direction with great freedom. The animal is long and 

 narrow (15 mm. x 5 mm.), the foot being about 2 mm. broad. 

 When disturbed it presses the girdle firmly to the substratum and 

 elevates the midregion of the body, in this way exerting suction 

 and thereby adhering tightly to the rock or other surface. The 

 girdle is in fact the prime 'holdfast' organ in all the chitons, and 

 not the foot. During the process of exerting suction the foot of 

 Ischnochiton may be more or less clearly removed, in its mid 

 portion, from all contact with the supporting surface, and it ex- 

 hibits also a type of reaction which is not without special inter- 

 est : the foot shows a decided tendency to fold together longitud- 

 inally, a deep depression appearing along its middle, giving it a 

 'ditaxic' appearance. Usually, in creeping, one retrograde pedal 

 wave is present on the foot. Ischnochiton does not carry out 

 pivoting movements so readily as do some other chitons, but 

 in contrast with them does move posteriorly in a 'spontaneous' 

 way for considerable distances, although it does subsequently 

 pivot or turn in a circular path if forced to move by continuous 

 directive stimulating agencies.^ During posterior locomotion the 

 retrograde character of the pedal wave is retained, as Olmsted 

 ('17) found to be true in Chiton tuberculatus. Olmsted forced 

 C. tuberculatus to creep posteriorly by attaching the posterior 

 end of the foot to a glass plate. Ischnochiton, however, moves 



^ This type of behavior is of special interest for the analysis of the phototro- 

 pism of Ischnochiton. It might be remarked that in the present species without 

 careful inspection there is some difficulty in distinguishing in dorsal view anterior 

 and posterior ends, owing to the indistinct sutures between the valves. Hence 

 it would appear that the lack of pronounced external signs of polarity is cor- 

 related with ready locomotion in either anterior or posterior direction. However, 

 there is not the slightest reason to suspect that any ethological significant is 

 involved in this correlation. 



