Sensory Discrimination: the Chemical Sense 73 



around that body in just the same way as they would were they 

 expanded in calm water " (366, p. 48), although the solid 

 stimulus is decidedly less intense than that offered by the 

 bubbles. Similarly, Fleure and Walton find that certain 

 species show little reaction to accidental contact with a pebble 

 that is moved, but react quickly to a finger (127). 



The body of a typical medusa or jellyfish consists of a bell- 

 shaped " umbrella " from the edge of which tentacles depend. 

 Hanging from the middle like the clapper of the bell or the 

 handle of the umbrella is the manubrium, at the end of which 

 is the mouth. In the medusa Carmarina hastata no differen- 

 tiation in reaction to contact and food stimulation appears, 

 merely a readier response of the tentacles to the latter ; but 

 we do find whatever evidence for the existence of a specific 

 sensation quality is furnished by localized sensitiveness, for 

 the skin of the under side of the umbrella, and of the manu- 

 brium, is very sensitive to mechanical stimulation, and wholly 

 insensitive to chemical stimulation, while the tentacles, as 

 has just been stated, react, by shortening and twisting them- 

 selves about the object, more readily to chemical than to 

 mechanical stimulation. A mechanical stimulus applied to 

 any part of the under edge of the umbrella produces after 

 from one to three seconds a movement of the manubrium tip 

 toward the point stimulated (289, 291). 



The little medusa Gonionemus murbachii (Fig. 8) shows, on 

 the other hand, two well-defined different responses to special 

 stimulation : motor reactions and food-taking reactions. The 

 motor or swimming reactions are given in response to me- 

 chanical stimulation and to the presence of food near the 

 animal in the water; but the food-taking reaction occurs 

 only in response to food (solution of fish meat) ; very rarely 

 a weak inorganic chemical stimulus will produce the begin- 

 ning of the response. An important exception to the usual 



