296 W. J. CROZIER AND LESLIE B. AREY 



ized nerve terminals in the integument is no objection to this 

 view; phj^siologically, no other interpretation is possible, and even 

 if free nerve terminals were the only form of peripheral nervous 

 organs they would have to be regarded as coming into association 

 with epithelial cells so specialized as to be capable of differential 

 excitabihty. The receptor distinctions here found are quali- 

 tative in character, cannot be referred to the quantitative dif- 

 ferences between the action of the several sources of excitation 

 (light, touch, etc.), and afford no support to the theory of gen- 

 eralized sense organs open to homologous activation by a 

 diversity of means. 



IV. THERMAL EXCITATION 



The relation of thermal conditions to the effectiveness of other 

 sources of activation (mechanical, photic, or chemical), and to 

 the animal as a whole, is of interest in connection with the possible 

 operation of heat and cold as specific activating agents. We 

 have not investigated the possibility that there may occur in 

 Chromodoris seasonal variations in thermal sensitivity, the 

 following tests being concerned with individuals collected in 

 shallow water during June and July. 



a. When placed in sea-water cooled to 15°C., Chromodoris remains 

 relatively quiet. Tactile responses are elicited without more than a 

 slight decrease in irritability. During the winter months, however, 

 when kept constantly at a temperature of about 17°, tactile responses 

 are apparently of lower amplitude than during the summer; the ani- 

 mals also move about somewhat less freely. 



After two or three minutes subsequent to immersion in water at 10°, 

 the animal remains stationary; responses to touch are very feeble. 

 The gill crown tends to remain partly contracted within its collar. A 

 few general contractive movements of the body are seen when the ani- 

 mal is first suddenly transferred to water of this temperature, but, 

 since they were not evidenced when the water is slowly cooled, down 

 to 10° (ten minutes for the decrease from 26° to 10°), and since they 

 are seen also at 15° when the nudibranch is suddenly transferred to 

 water at that temperature, it is only on the basis of the gill reaction 

 that water at 10° can be held to be stimulating. 



At 4° to 5° the 'rhinophores' and the gills remain contracted, usu- 

 ally completely so. The whole body is ordinarily contracted to a con- 

 siderable extent, after a few preliminary movements of extension and 

 slow longitudinal contraction. After five minutes, tactile responsive- 



