210 LESLIE B. AREY AND W. J. CROZIER 



eral way true. If the nervous connection of one ctenidial wall 

 of the foot mass by means of the paUial cord be severed, then 

 the muscles on that side, passively stretched, or compressed, 

 could not (or might not be able to) get readily into nervous 

 communication with the rest of the body; hence we should under 

 these conditions expect geotropic movements to be imperfect. 

 No tests have yet been made to discover the effect of uni- or of 

 bilateral section of the pallial nerve cord on horizontal creeping. 



6. Summary 



The general surface of Chiton, aside from the tegmental sur- 

 faces, is actively responsive to touch. The aesthetes upon the 

 shell valves are not sensitive to tactile stimulation. The foot is 

 negatively reactive to small surfaces, positively thigmotactic to 

 large surfaces. Chiton is negatively geotropic, and also nega- 

 tively rheotropic to currents of some strength ; these two modes 

 of response appear to depend respectively upon the develop- 

 ment of unequal tensions in the musculature, owing to the 

 weight of the body, and upon the local deforming influence of a 

 water stream — they result from stimulation through deforming 

 pressure. 



Although much has been written upon the morphological 

 character of the sensory organs in Chiton and upon the general 

 structure of the integument (Blumrich, '91 : Plate '87-01 a), there 

 has been very little in the way of experimental evidence bearing 

 upon functional relations. Heath ('03) thought that the pro- 

 boscis of Cryptochiton stelleri might exercise tactile as well as 

 gustatory functions, and described some feeding experiments in 

 support of this idea. From the foregoing account it will be 

 seen that tactile (contact) irritability is a general integumentary 

 function, leading to responses of an 'advantageous' character. 

 The receptors concerned are apparently of several kinds: 1) 

 those upon such surfaces as the foot, mouth area, and gills, 

 and, 2) those associated with the tubercles and 'hairs' upon 

 the girdle. The former are probably of a more generalized 

 character than the latter, but, for some parts at least, the propo- 



