74 



The Animal Mind 



inefficacy of mechanical stimuli in bringing about the feeding 

 reaction occurs when a moving mechanical stimulus is used ; 

 this very quickly produces the early stages of the food-taking 

 response. Special reactions to stimuli in motion are wide- 

 spread throughout the animal kingdom; their significance 

 will be discussed in the chapter on Space Perception. The 

 food-taking response in Gonionemus shows a marked coor- 

 dination 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 envelops the 

 food with its lips (451). 

 Another ccelenterate 

 whose reactions to chemi- 

 cal stimulation have been 

 observed is the cteno- 



FIG. 8. -Gonionemus. After Hargitt. ph()re 



body is an elongated oval, with longitudinal ciliated ridges, 

 having the mouth slit 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 will 

 be considered later. The aboral region is far more sensitive 

 than any other to mechanical stimulation ; the slightest 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 

 sensitive 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 



