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Show 1884.] DURING THE VOYAGE OF H.M.S. 'CHALLENGER.' 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 necera, 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. Bathynomus giganteus, a genus allied to the Cymathoadae, 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 Serolis, 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 considerably 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. U p 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 neara 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 neara 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 contour * from the lower end of this mass of cells a stout nervous |
