124 The Animal Mind 



taken from illuminated spots, has been shown both for sea- 

 anemones and for Hydra (127). 



Many Medusae or jellyfish also react to light more slowly 

 than to other forms of stimulation. It is true that on Sarsia, 

 a form tested by Romanes many years ago, light seemed to 

 act as quickly as any other stimulus. If a flash of light were 

 allowed to fall on the animal while it was moving about, 

 "prolonged swimming movements" ensued; if it was at rest, 

 it gave only a single contraction another instance of the 

 effect of physiological condition upon reaction. Sudden 

 darkening produced no reaction, whence Romanes concluded 

 that "it is the light per se and not the sudden nature of the 

 transition from darkness to light which in the former experi- 

 ment acted as the stimulus." There are, however, as we 

 shall see, other animals in which an increase of illumination 

 brings about response where a decrease fails, and vice versa. 

 When a beam of light was thrown into a bell- jar containing 

 many Sarsiae and placed in a dark room, "they crowded into 

 the path of the beam 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" (365, 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. 

 Tiaropsis, 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 depend- 

 ence of response on a special organ indicate that light may 

 produce a specific sensation, always granting the presence of 



