148 CRUSTACEA 



upon the absorption of a great number of the light -rays by 

 pigment, and the transmission of only a limited number to the 

 sensory surface, is only possible when there is a strong light, 

 and there is no need for economising the light-rays. The most 

 important discovery was made by Exner,^ that the majority of 

 animals with compound eyes had the power of so arranging the 

 pigment in their eyes as to enable them to see in two ways. 

 In bright light the pigment is situated as in Fig. 103, A, so as 

 completely to isolate the rhabdoms from one another (day- 

 position) ; but in the dusk the pigment actively migrates, the 

 irido - pigment passing to the surface (B) near the tops of 

 the crystalline cones, and the retino-pigment passing interiorly 

 to rest on the membrana fenestrata at the bases of the rhabdoms 

 (night-position). When this happens the rays of light which 

 strike the ommatidia at all sorts of angles, instead of being 

 largely absorbed by the pigment, are refracted by the crystalline 

 cones and distributed over the tops of the rhabdoms, passing 

 freely from one ommatidium to another. In this way the eye 

 acts on this occasion, not by mosaic vision, but on the principle 

 of refraction, as in the Vertebrate eye. Of course the distinct- 

 ness of vision is lost, but an immense economy in the use of 

 light-rays is effected, and the creature can perceive objects and 

 movements dimly in the dusk which by mosaic vision it could 

 not see at all. The pigment is contained in living cells or 

 chromatophores, and it is carried about by the active amoeboid 

 movements of these cells with great rapidity. 



Now, besides the active adaptability to different degrees of 

 light brought about in the individual by these means, we find 

 Crustacea living under special conditions in which the eyes are 

 permanently modified for seeing in the dusk, and this naturally 

 occurs in many deep-sea forms. 



Doflein " has examined the eyes of a great number of deep- 

 sea Brachyura dredged by the Valdivia Expedition, and as the 

 result of this investigation he states that the eyes of deep-sea 

 Brachyura are never composed of so many ommatidia, nor are 

 they so deeply pigmented as those of littoral or shallow water 

 forms. At the same time an immense range of variation occurs 

 among deep-sea forms which are apparently subjected to similar 



' Die Physioloyic der facetticrten Augen von Krcbsen und Insectcn. Leipzig, 

 Wien, 1891. 2 r„/j.ii.[n Expedition, vol. vi., 1904. 



