THE SENSORY RESPONSES OF CHITON 187 



except when it is creeping over a sharply curve surface. There 

 seems to be a pronounced tendency for the avoidance of un- 

 even tensions among the muscles. The normal place of resi- 

 dence is upon a flat surface ; the somewhat unexpected flexibility 

 of the body is nevertheless important, since it enables the girdle 

 and foot to remain, during creeping, in close contact with the 

 substratum, even though the latter be quite irregular. 



The flexibility of the body is more evident in a form such as 

 Ischnochiton purpurascens. This animal is long and narrow. 

 It creeps with unexpected freedom, drops from one rock surface 

 to another, when stimulated by light, and rights itself easily. 



3. Locomotion 



The locomotor activities of Chiton demand a few words at this 

 point, and we are able to add slightly to previous descriptions of 

 its pedal movements. C. tuberculatus characteristically pro- 

 gresses in an anterior direction. This is accomplished by means 

 of pedal waves which are of a retrograde character, coursing from 

 anterior to posterior as the chiton advances (Parker, '11, '14); 

 in this respect it resembles another placophoran studied by Vies 

 ('07). As Parker observed, however, C. tuberculatus can also 

 make backward movements of limited extent. Olmsted showed 

 ('17 a) that in these backward movements of chiton the retro- 

 grade direction of the pedal wave is retained, as is also true in the 

 Fisurella which Olmsted forced to creep posteriorly for a short 

 distance; this we can confirm both for the pedal wave in chitons 

 constrained to creep posteriorly, as in Olmsted's experiment, by 

 having merely a small part of the posterior region of the foot 

 attached to a substratum, and also for the occasional backward 

 movements which occur when the whole foot is attached. Lat- 

 eral waves, or at any rate one lateral wave-like movement at a 

 time, are produced on the foot when the animal is intensely 

 stimulated on one side (Parker, '14) ; in this case we find that the 

 pedal wave courses from the unstimulated to the stimulated side 

 (i.e., it is retrograde), but it not noticeably lifted from the sub- 

 stratum in wave form. Similar movements appear at the ante7 



