212 LESLIE B. AREY AND W. J. CROZIER 



Chiton exhibits mouth movements similar to those figured by 

 Heath ('03) for Cryptochiton. It is questionable whether these 

 movements originate normally from tactile excitations. On a 

 glass surface they are never observed except in starved indi- 

 viduals. The mouth opens widely, permitting the subradular 

 organ to be pushed forward. In a starved animal these move- 

 ments, which occur in rhythmic series, may be initiated by 

 causing a bubble of air to become entrapped in the depression 

 surrounded by the lips. The radula is never thrust forward 

 sufficiently to come clearly into view. Full feeding movements 

 are obtained, however, when a spUnter of the intertidal rock 

 surface is placed in contact with the proboscis. It is of course 

 possible, as Heath suggests, that the rich supply of nerve endings 

 on the surface of the subradular organ is in part of tactile 

 significance. 



V. THERMAL EXCITATION 



1. Behavior at different temperatures 



It is worthy of remark that very few marine invertebrates, if 

 indeed any, seem to possess a well-developed temperature sense. 

 Although as yet we know little of the integumentary senses in 

 marine annelids, Crustacea, or mollusks (Kafka, '14), we 

 may note that in echinoderms (Crozier, '15 b; Olmsted, '17 b) 

 sensory discrimination for heat and cold is very weak, and 

 that even in Amphioxus, where there are good indications of 

 possibly both heat and cold receptors (Parker, '08, p. 430), the 

 responses of the animal to either heat or cold are by no means 

 of that delicately sensitive character which we usually asso- 

 ciate with photic, tactile, or chemical receptors. We believe 

 this to indicate a poorly developed sensory mechanism for heat 

 and cold reception. There is little reason to regard this con- 

 dition as one of adaptation to, or correlation with, life in a 

 situation where thermal receptivity would not be valuable, 

 owing to the small range of temperature occurring in the habi- 

 tat of the creatures in question. It would be equally sensible 

 to consider that, if the animals concerned had developed deli- 



