1884.] DURING THE VOYAGE OF H. M.S. 'CHALLE^GER.' 339 



antarctica. Of both these species the specimens from the more 

 southern latitudes and deeper water are larger than those found to 

 the north and in comparatively shallow water ; especially is this so 

 with Serolis bromleyana. Comparing the deep-sea species with those 

 from shallow water, it must be noticed that in two species from the 

 deep sea, Serolis bromleyana and Serolis neara, the genus attains 

 to its largest size ; these two species are indeed among the largest of 

 the whole group Isopoda. Another deep-sea Isopod described by 

 Milne-Edwards, viz. Bathynomns giganteus, a genus allied to the 

 Cymathoadse, is also of colossal size ; and the 'Challenger' collection 

 contains a new deep-sea species which evidently belongs to the same 

 family, and is also extremely large. All the deep-sea species of 

 Sei-olis, with the exception of Serolis antarctica, have the epimera, 

 especially those of the sixth pair, greatly elongated to an extent not 

 found in any of the species inhabiting shallow water. 



The chief differences, however, which are noticeable between the 

 shallow-water and deep-sea representatives of the genus are to be 

 found in the eyes. 



It is well known that many deep-sea animals are entirely deprived 

 of eyes, while in others (notably many fishes) the eyes are consider- 

 ably enlarged though often pale in colour, owing to the partial absence 

 of pigment. The presence of eyes in deep-sea animals, which would 

 almost seem to be useless to their possessor, has been accounted for 

 by the theory of " abyssal light," by which it is supposed that the 

 light emanating from phosphorescent Alcyonarians is sufficient to 

 enable these animals to see. Exact researches into the structure of 

 eyes in the majority of deep-sea animals are wanting, and it is not 

 certain how far optical structures are present. Up to the present 

 the only comparison of the minute structure of the eyes in shallow- 

 water and deep-sea representatives of the same group is, so far as I 

 am aware, to be found in Dr. Hoek's Report on the 'Challenger' 

 Pycnogonida. It appears from his results that the eyes in the deep- 

 sea species are sometimes altogether absent, sometimes furnished 

 with well-developed retinal structures ; in some forms the eyes " have 

 a distinct lens — a rounded spot marked by its brightness" ; but they 

 are quite destitute of pigment, and instead of a retina the cavity of 

 the eyes is filled with a mass of connective tissue. 



In Serolis the conditions are rather different; in none of the 

 species from the deep sea is there a retinula developed, at least 

 nothing at all similar to the retinula of the species from shallow 

 water, but the vitreous body is represented. In Serolis necera the 

 cornea is distinctly faceted, and corresponding to each facet is an 

 oval body which appears to represent a vitreous body ; instead of 

 being clear and transparent like the highly refractive vitreous bodies 

 of the shallow-water Serolis and other Isopoda, this structure in 

 Serolis necera is granular and rather opaque in appearance, the 

 upper half is encircled by a ring of pigment. The lower end of the 

 " vitreous body " is imbedded in a mass of cells, which are small and 

 closely pressed together, assuming in consequence a hexagonal con- 

 tour; from the lower end of this mass of cells a stout nervous 



