Sensory Discrimination: Vision 137 



tensity is found, although in the hydroid colonies of Tu- 

 bularia it appears to be wholly lacking (564). Many 

 sea-anemones are wholly unaffected by Hght stimulation, 

 Sagartia luciae and Metridium, for example (286). Many 

 others have been observed to contract when the light 

 intensity is increased (266, 374, 521). Eloactis producta 

 expands its tentacles only in light ot low intensity, taking 

 about fifteen minutes to do so when covered with' a hood, 

 and retracting in five minutes when the Ught is restored. 

 This retraction is decidedly slower than that produced 

 by mechanical stimulation (286) ; thus we have some 

 evidence that it is accompanied by a specific sensation qual- 

 ity. That the responses to light are more marked in ani- 

 mals which have been living in comparative darkness than 

 in those taken from illuminated spots, has been shown 

 both for sea-anemones and for Hydra (228). 



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 hght 

 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 in- 

 stance of the effect of physiological condition upon reac- 

 tion. 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 experiment 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 Hght was 

 thrown into a bell-jar containing many Sarsiae and placed 

 in a dark room, "they crowded into the path of the beam 



