THE SENSORY RESPONSES OF CHITON 253 



innervation, and constitute, in Pelseneer's opinion, special sen- 

 sory regions, each of them being protected by the ventral face 

 of the papilla, which is sometimes spiculose. We have described 

 how the respiratory water current impinges on the dorsal face of 

 each papilla. The conditions are therefore favorable for the sit- 

 uation of an organ ''testing the quality of the water." The 

 papilla may be significant for egg-laying responses (i.e., in the 

 reception of a stimulus provided by the spermatic fluids), as it 

 seems more prominent in the females. This remains to be 

 tested. Copeland ('18) has brought forward good evidence that 

 the osphradium of carnivorous snails is concerned in the recep- 

 tion of chemical excitation by dilute solutions of materials ema- 

 nating from food; his further contention, that this organ is 

 therefore an 'olfactory organ/ because the exciting agent is 

 very dilute, is an unnecessary metaphor (cf. Arey, '18 b). 



VIII. THE NERVOUS SYSTEM AND SENSE ORGANS OF CHITON 



The main features of the foregoing account may now be briefly 

 summarized. This report makes no claim to completeness; it 

 does lay a solid foundation for further investigation in at least 

 two directions: 1) the phenomena of seeming adaptation in the 

 ethology of chiton, and, 2) the physiology of certain types of 

 irritability. The sensory conditions are here unexpectedly com- 

 plex. The major pathways of nervous transmission are, by 

 contrast, unusually clear and well > defined. The manner in 

 which sensory capacities and modes of reaction are involved in 

 the complex determination of natural behavior can be followed 

 in great detail. 



a. Tactile receptors are absent from the shell surfaces. The 

 'scales' and 'hairs' upon the girdle are important tactile organs. 

 The ctenidia are also sensitive to touch, as are the proboscis, 

 the foot, and the ventral surface of the girdle. The foot is posi- 

 tively thigmotactic to large surfaces, but retracts locally when 

 stimulated by a small surface. 



The tegmental aesthetes are photosensitive; they are activated 

 by light of constant intensity and by sudden decrease in light 



