Sensory Discrimination: The Chemical Sense y'j 



sponse in Gonionemus shows a marked coordination of 

 movements; if the food touches one or more tentacles, 

 these contract and twist about it ; they then bend toward 

 the manubrium, and the margin of the bell also bends 

 in; the manubrium swings over toward the bell and en- 

 velops the food with its lips (802). 



Another coelenterate whose reactions to chemical stimu- 

 lations have been observed is the ctenophore Beroe ovata. 

 Its body is an elongated oval, with longitudinal cihated 

 ridges, having the mouth sHt at the end which is normally 

 uppermost when the animal is at the surface of the water, 

 and at the opposite end an otolith or statolith organ lying 

 between two flattened "polar plates." The significance of 

 this organ wUl be considered later. The aboral region is 

 far more sensitive than any other to mechanical stimu- 

 lation ; the sKghtest touch on one of the polar plates causes 

 the animal to shorten itself and fold in the plates. The 

 aboral end, being the hind end of the creature, is not 

 usually brought into contact with objects. Nagel, who 

 studied the animal, suggests that this region, being sensi- 

 tive to changes in pressure, may enable the animal to right 

 itself when it rises to the surface with the aboral end up, 

 as the change from water to air pressure could not fail to 

 stimulate the polar plates. Nagel apparently made no 

 experiments on the behavior of Beroe with reference to 

 food stimuli ; for chemical stimulation he used picric acid, 

 dilute hydrochloric acid, quinin, strychnin, saccharin, 

 coiunarin, vanillin, and naphthalin. To all these un- 

 wonted stimuli the animal responded by some form of 

 negative reaction, indicating possible unpleasant feeling. 

 The edges of the mouth, where the nerves end in bulb-like 

 structures, reacted to quinin, vanillin, and coumarin by 

 stretching the mouth into a circular form instead of its 



