186 LESLIE B. AREY AND W. J. CROZIER 



girdle is then closely applied to the substratum, and although, 

 while under water, a respiratory current may be demonstrated, 

 no sand grains gain admittance to the gills. The riblets and tiny 

 channels normal to the girdle margin are important in this con- 

 nection. If during creeping a lightly sanded spot is encountered, 

 the girdle acts immediately as a plough, causing the sand to be 

 pushed to one side. Although the tactile response of the girdle 

 is thus very delicately adjusted, the commensal isopods (p. 167) 

 are able to insinuate themselves beneath it without (usually) 

 inducing any response. 



The 'rolling up' of the body is the activity of chiton most 

 frequently mentioned in descriptions. The animal when de- 

 tached from the rock, even in the case of the smallest specimens, 

 usually bends the head end sharply ventralward, the curvature of 

 the posterior end following, so that the body becomes ultimately 

 rolled together, the anterior edge of the girdle sometimes being 

 beneath the posterior extremity, at other times the two ends 

 being simply in close contact. 



This response might conceivably be of significance in the life of 

 chiton. When placed upon its dorsum on a smooth surface, it 

 is impossible for C. tuberculatus to right itself. When rolled 

 together, however, it could easily be moved by wave action to a 

 location more favorable for righting. Moreover, the dorsal sur- 

 face of the valves being sharply arched in the mid-line, the ani- 

 mal automatically rolls over to one side. This results in righting 

 behavior somewhat similar to that evidenced by Holothuria 

 (Crozier, '15 b). That it is ever actually resorted to in nature is 

 quite improbable. It seems merely that the ' curling' is an un- 

 natural result of the tendency to maintain the foot in contact 

 with the substratum, its protective appearance and functional 

 value in righting being illusory. 



The flexibility of the body shown in 'curling up' is also evi- 

 dent in other movements. Although the plates are closely ar- 

 ticulated, some sidewise bending is nevertheless possible. The 

 animal may also become arched dorsally to a considerable extent, 

 as well as bent sharply in the ventral direction, at any level. 

 This flexibility is rarely shown in the natural habitat of chiton 



