SENSORY REACTIONS OF CHROMODORIS ZEBRA 295 



Some other nudibranchs, as Hermaea and certain Aeolids 

 (Garstang, '90, p. 423), and Facelina goslingi, are also reactive 

 to shading. In FaceUna the anterior end is the sensitive part. 

 It responds to decreased hght intensity by a quick retraction of 

 the head, followed by its rapid reextension. The head end must 

 itself be shaded in order to produce this effect. The response 

 occupies 0.6 to 1.0 second, and is followed by a 0.5 to 1.0 minute 

 refractory period during which no shading response can be 

 elicited. It is very difficult to fatigue this reaction, probably 

 owing to the long refractory period; after twelve to fifteen suc- 

 cessive shadings and reactions there was no evidence of exhaustion. 



3. Discussion 



On the ground of their respective modes of distribution upon 

 the surface of the animal, it would appear that tactile and photic 

 irritability are served by quite distinct receptive mechanisms. 

 Especially is this so in the case of shading. After complete ex- 

 haustion of the shading response, as well as during the brief 

 refractory periods in which shading is non-effective, tactile irri- 

 tability of the gill plumes seems not in any way impaired. Since 

 precisely similar motor effects are concerned, we may conclude 

 that exhaustion of the receptors for shading does not interfere 

 with tactile irritability, and hence that the receptive mechanisms 

 are respectively distinct even upon the gill plumes. 



Shading and light of constant intensity also act upon diverse 

 receptors, since the reactions they induce are spatially separate 

 and quite distinct in character. If the eyes of C. zebra are pho- 

 tosensitive, we must conclude that they are not the only photo- 

 receptors concerned in the activating effect of light of constant 

 intensity, as is shown by the behavior of the gill crown. The 

 'rhinophores' are preeminently sensitive to touch, but are not 

 reactive to light. 



Speaking in general terms, we may, then, distinguish at least 

 three classes of differentiated receptors in Chromodoris: tactile 

 organs, light-detecting organs, organs sensitive to sudden dimi- 

 nution of light intensity. The absence of morphologically special- 



