196 Mr. S. I. Smith on the Abyssal 



species inhabiting different depths where the eyes of the deeper- 

 water species are much the smaller ; for example : Sympa- 

 gurus pictuSj 164 to 264, and Parapagurus pilosimamis, 250 to 

 2221 fathoms ; Pontophilus gracilis, 225 to 458, and P. abyssi, 

 1917 to 2221 fathoms ; and Nemaiocarcinus cursor, 384 to 838, 

 and N. ensiferus, 588 to 2033 fathoms. 



In a large number of deep-water and abyssal species the 

 ocular pigment is dark purplish, brownish, reddish, light 

 purplish, light reddish, or even nearly colourless, while the 

 number of visual elements may be either very much less or 

 very much greater than usual. The eyes of the species of 

 Glyphocrangon and of Benthonectes are good examples of 

 highly developed eyes of this class. In many cases the presence 

 of light-coloured pigment is accompanied with reduction in 

 the number of visual elements precisely as in black eyes, 

 Parapasiphae sulcatifrons, P. cristata, Acanthephyra microph- 

 thalmia, and the species of Hymenodora being good ex- 

 amples. 



In other cases there are apparently radical modifications in 

 the structural elements of the eye without manifest obsoles- 

 cence. The large and highly-developed but very short-stalked 

 eyes of the species of Glyphocrangon, apparently specialized for 

 use in deep water, probably represent one of the earlier 

 stages of a transformation which results finally in the oblite- 

 ration of the visual elements of the normal compound eye and 

 the substitution of an essentially different sensory structure. 

 In Pontophilus abyssi the transformation has gone further ; 

 the eyes, though fully as large as in the allied shallow-water 

 species, are nearly colourless, not very distinctly faceted, and 

 have probably begun to lose the normal visual elements over 

 a portion of the surface. In the eyes of several of the species 

 of Munidopsis the normal visual elements have entirely dis- 

 appeared, and there is an expanded transparent cornea backed 

 by whitish pigment and nervous elements of some kind. I 

 am well aware that there is as yet no conclusive evidence that 

 these colourless eyes are anything more than the functionless 

 remnants of post-embryonic or inherited organs ; but the fact 

 that in some species they are as large as the normal eyes of 

 allied shallow-water forms is certainly a strong argument 

 against this view. In the species of Pentacheles there is 

 still better evidence that the eyes are not functionless ; for, 

 although they have retreated beneath the front of the cara- 

 pace, they are still exposed above by the formation of a 

 deep sinus in the margin, and the ocular lobe itself has thrown 

 off a process which is exposed in a special sinus in the ventral 

 margin. It is easy to conceive how these highly modified 

 eyes of Pentacheles may have been derived from eyes like those 



