138 The Animal Mind 



and were most numerous at that side of the jar which was 

 nearest the light." " There can thus/' concludes Romanes, 

 "be no doubt about Sarsia possessing a visual sense" 

 (641, p. 41). But as these reactions are not differentiated 

 in any way, they cannot be taken as evidence of a specific 

 sense, unless indeed they depend on a specialized sensory 

 structure. This latter Romanes found to be the case; 

 Sarsia has pigment spots on the margin of its bell, and its 

 response to light ceased when these were destroyed. Tiar- 

 opsis, another jellyfish studied by the same observer, 

 gave further evidence of "a visual sense" in the fact that 

 it responded to light more slowly than to mechanical 

 stimulation. In Gonionemus, both difference in reaction 

 time and dependence of response on a special organ indi- 

 cate that light may produce a specific sensation, always 

 granting the presence of consciousness. Yerkes found that 

 this jellyfish, unlike Sarsia, reacts in the same manner in 

 passing either from sunlight to shadow or the reverse. In 

 both cases it stops swimming and sinks to the bottom. A 

 sudden change of illumination, therefore, checks its activity. 

 On the other hand, if when the light falls upon it the ani- 

 mal is at rest, it becomes active again; but sudden de- 

 crease of illumination has no effect upon the resting animal. 

 The inhibitory effect of strong light falling upon the jelly- 

 fish while in motion Yerkes explains as a special adapta- 

 tion. For one case of such increase of illumination occurs 

 when the animal swims, bell upward, to the surface on 

 being disturbed ; the light of the surface is of course nor- 

 mally stronger than that in the lower regions. The in- 

 hibition of activity resulting causes the animal, after 

 turning over, to sink slowly, bell downward, with expanded 

 tentacles. This is a position that gives it a better chance 

 of catching food and carrying it to the lips than is offered 



