STIMULATIONS IN NUDIBRANCHIATE MOLLUSKS 431 



working the oral apparatus sucking in the mixture. Applying 

 broth on the back it turns around, as if searching for the stimulus. 

 The reaction for broth of sea-urchin gonads is twice as strong as 

 for that of jellyfish. The response was from + to + + +, but 

 most effective for the sea-urchin's. The oral tentacles showed a 

 most definite positive reaction toward the stimulus. Touching 

 the tentacle on the left or right side, I could lead the animal at 

 will from one side to the other; it always turned after a short 

 interval of time toward that side which tentacle was stimulated. 

 This is equally definite when using a piece of sohd food which 

 it eats readily, e.g., gonads of sea-cucumber or sea-urchin. The 

 reaction as shown by Hermissenda toward distance stimuli is 

 less definite than that toward contact stimuli. Receptors for 

 distance stimuli seem to be present in the oral tentacles; but 

 receptors for contact stimuli, judging by the mode of response, are 

 more specific than are the distance receptors. Contact receptors 

 are present all over the body, perhaps also chemoreceptors, but 

 the latter are specialized in the oral tentacles, as they seem to 

 be used in discriminating between foods. They may, therefore, 

 be gustatory in function, because the animal may be led about, 

 from side to side, in the dish by touching the oral tentacle by some 

 palatable food ; that is, a food which the organism feeds on readily. 

 No such results are effected by treating the dorsal tentacles, the 

 so-called rhinophores, in the same way. The head, in general, 

 seems to be most sensitive to stimuli, tactile or otherwise. When 

 food stimuli are applied to the head, the animal turns in various 

 directions so as to search for them. The dorsal tentacles do 

 not respond to distance stimuli ; their response to a food stimulus 

 is similar as that given to a tactile stimulus, e.g., a clean glass rod. 

 Hermissenda does not have the ability to locate sohd foods; it 

 really seems that the animal comes upon the foods accidentally. 

 It is apparent that the animal can taste food in solution with most 

 parts of its external surface, and particularly that of the head and 

 the oral tentacles; but it does not look as though the organism 

 is capable of scenting its food. I am not able to say that the 

 dorsal tentacles are used for this purpose. I repeatedly found 

 hungry animals within ca. 1 cm. from food they readily ate when 



