480 A. M. Banta 



sive to mechanical stimulation, this difference appearing in the 

 number and vigor of its responses and in its having a decidedly 

 lower threshold of stimulation than Asellus. 



3. Caecidotea, though at first less rhetotactic than Asellus, is 

 persistently rheotactic, whereas rheotaxis with Asellus is only 

 temporary. When the stimulus of strong illumination in the 

 lower part of the trough was added to the stimulus of a current, 

 the Caecidotea more persistently remained within the upper dark 

 end. 



Although there are considerable differences between the re- 

 sponses of the two species in minor details, these are, after all, 

 not so much differences of kind as of degree. Physiologically 

 considered, the two species are very much alike. 



The relatively slight sensitiveness to light shown by Caecidotea, 

 as compared with Asellus, is what one might expect from the 

 responses of eyeless animals in general, as compared with those 

 which possess eyes, For example, Dubois ('89, pp. 358-359) 

 found that Proteus, the blind salamander of the European caves, 

 responded to light from a projection lantern. Semper ('89, p. 79) 

 states that Proteus is sensitive even to daylight. Eigenmann 

 ('00, pp. 113-116) found the blind fish, Ambl}opsis, sensitive to 

 strong light suddenly thrown upon it as well as to diffuse daylight, 

 for these animals, when kept in a pool in the open air, where they 

 remained concealed among rocks during the day, swam about 

 freely in twilight and at night. It has been shown by Payne, 

 ('07) that Amblyopsis reacts to light from a "100 c.p." acetylene 

 lamp at 32 inches from the end of the aquarium; but with this 

 intensity of illumination an average of only 54 per cent of the 

 individuals were in the end farther from as against 46 per cent in 

 the end nearer to the light. With considerably greater intensi- 

 ties he found the reactions were much more decided. Hence it 

 seems that the lower intensity used by Payne ('07, p. 318) is near 

 the threshold of stimulation for Amblyopsis. 



There is no reason to suppose from any of these observations 

 that such animals respond to low intensities of illumination. * 



