234 LESLIE B. AREY AND W. J. CROZIER 



movements a local sensitivity to illumination, as likewise the 

 anterior edge of the proboscis probably does. Heath ('99, p. 579 ; 

 sep. p. 4) thought the proboscis of Ischnochiton to be sensitive 

 to light. 



In the adult Chiton there are no detectable cephalic eyes, 

 although in some small species the larval 'eye' may persist into 

 adult life (Heath, '04 b) . The larva of Ischnochiton magdallensis 

 is positively phototrophic (Heath, '99, p. 637; sep. p. 62). 



b. For shading. The edge of the girdle and the tegmental 

 surfaces of the valves are sensitive to decrease in light. In the 

 former case the nature of the receptors is obscure; in the latter 

 case they may or may not be identical with the organs sensitive 

 to the constant action of light. The minimal area which must be 

 shaded in order to effect a response is very small. The shadow 

 of a fly 6 feet distant in moderately bright sunlight induces 

 violent shading responses. An opaque spot 2 mm. in diameter 

 on a glass plate at a distance of about 2 cm., casts sufficient 

 shadow upon a valve to lead to pronounced reactions. The 

 threshold of sensitivity is very low, an almost imperceptible 

 decrease in light intensity being quite effective. 



The arrangement of the aesthetes in the tegmentum is such 

 that many micraesthetes accompany each megalaesthete ; it 

 might be supposed that the micraesthetes are of different re- 

 ceptive value, but no proof for this can be given. 



The physiological distinctness of the shading receptors on the 

 ventral edge of the girdle is shown by the fact that their complete 

 exhaustion by repeated activation does not interfere in the least 

 with tactile responses. 



c. For increased illumination. The great sensitivity of chiton 

 to shading and the poorly developed character of the responses 

 to increased illumination make adequate experimentation very 

 difficult. The edge of the girdle is the most sensitive part; 

 probably the surface of the proboscis and palp are also sensitive. 

 Nothing more can at present be said under this head. 



