148 CRU SiG E Ate as CHAP. 
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 lght, 
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 lttoral or shallow water 
forms. At the same time an immense range of variation occurs 
among deep-sea forms which are apparently subjected to similar 
1 Die Physiologie der facettierten Augen von Krebsen und Insecten. Leipzig, 
Wien, 1891. 2 Valdivia Expedition, vol. vi., 1904. 
