200 LESLIE B. AREY AND W. J. CROZIER 



structures is well known. Definite evidence as to their functional 

 significance has been completely lacking. In addition to the 

 'eyes' (Moseley, '85; Plate, '99; Nowikoff, '09); micra- and 

 megalaesthete organs of varied form are present, and some of 

 them seem so constructed as to be (?) serviceable as tactile re- 

 ceptors. A function of this sort has in fact been somewhat 

 doubtfully suggested for them (Kafka, '14, p. 100). 



As already stated, however, the shell plates of adult chitons 

 seemed insensitive to touch. A slight pressure, however, de- 

 forms the tissue underlying the plate, and is sufficient to induce 

 a more or less pronounced sucking reaction. We tested there- 

 fore young C. tuberculatus, one to two years old, under the im- 

 pression that in older individuals the aesthetes might be de- 

 stroyed, as their cavities are exposed by the erosion of the 

 cuticula. The result was again negative so far as tactile sensi- 

 tivity of any part of the shell surface was concerned. 



We then employed a method which completely avoided me- 

 chanical depression of the tissues beneath the shell plate. The 

 free umbo ('beak') of the third or fourth valve was tightly 

 gripped between the jaws of a haemostat. This did not involve 

 damage to any of the soft parts. The forceps could then be 

 clamped to an upright. By means of a small hole through the 

 girdle or with the aid of cement attaching a thread to one of the 

 anterior valves, the movements of the chiton could then be 

 recorded graphically upon a kymograph paper. When the surface 

 of a valve rigidly held in this way was explored with a needle or 

 with a larger object, no tactile responses were elicited. (Shad- 

 ing must be avoided and rhythmic spontaneous contractions of 

 the animal must be discounted.) Single touches and a moving 

 point were alike without effect. There are no tactile receptors 

 in the shell plates. 



The histological nature of the sense organs in the tegmentum 

 varies considerably in different genera of chitons (Plate, '99). 

 Upon the shell of Ischnochiton there are minute, projecting 

 'hairs.' We find that the tegmentum of Ischnochiton pur- 

 purascens is very sensitive to touch. 



These results allow us to state that in all probability the sen- 



