PHOSPHORESCENT ORGANS AND EVES 



151 



number and long, the corneal lenses are highly arched, and the 

 pigment is reduced to a few clumps in tlie iris. This part of the 

 eye is evidently adapted for forming a vague superposition-image 

 in the dusk. The ventro-lateral part (B), on the other hand, is 

 composed of numerous small ommatidia, the crystalline cones of 

 which can be completely iso- 

 lated from one another by 

 the irido-pigment. Immedi- 

 ately below this part of the 

 eye is a phosphorescent organ 

 (C) provided with a lens and 

 tapetum. Chun suggests that 

 the ventro-lateral part of the 

 eye is used for obtaining a 

 clear mosaic image of ol;)jects 

 illuminated by the phos- 

 phorescent organ, while the 

 frontal part of the eye is 

 used for obtaining general 

 visual impressions in dimly 

 lit regions. This curious 

 differentiation of the eye 

 into two parts apparently 

 only occurs in predaceous 



animals, which capture their prey alive upon the bottom, and to 

 whom a clear vision of moving organisms is a necessity. 



Another instance of Crustaceans making use of their own 

 light is given by Alcock,^ who found two deep-sea prawns, Hetero- 

 carpus alpJionsi and Aristaeus coruscatis, at about 500 fathoms in 

 the Indian Ocean. These animals produce a highly phosphor- 

 escent substance which they eject from the antennary glands, and 

 they possess very large, deeply-pigmented eyes. 



The whole subject of the modification of the pigment and 

 structure of Crustacean eyes is an interesting one, because it 

 presents us with one of those cases in which the direct response 

 to a stimulus acting within the lifetime of the individual seems 

 to run parallel to the fixed adaptations of a whole species, which 

 have become hereditary and apparently independent of the 

 external stimulus of light or of the absence of light. As far 



' Lot: cit. p. 150. 



Fiu. 104. — Section of eye of Stylocheircrn viasti- 

 gophornm. A, Frontal portion ; B, ventro- 

 lateral portion ; C, phosphorescent organ ; 

 D, entrance of optic nerve ; c, corneal lens ; 

 (■/•, crystalline cone ; pg, pigment ; ret, 

 retiuula ; rh, rhabdoni. (After Chnn.) 



