Among the Isopoda we have several very characteristic forms—no fewer than nine distinct genera peculiar to the abysmal zone have been described by Beddard—and of these two, _Bathynomus_ and _Anuropus_, are to be regarded as types of sub-families. They seem to be very unevenly distributed over the floor of the ocean, some regions, such as the whole of the Central and Southern Atlantic and the Central and Western Pacific, produce none; whilst the waters of the east coast of New Zealand, the Crozets, and others, produce a great many varieties. Many of the deep-sea Isopoda exhibit characters that are usually associated with the bathybial life. Thus, according to Beddard, thirty-four of the deep-sea species are totally blind, and eighteen have well-developed eyes. In four species there are eyes which are evidently degenerating. If we compare, for instance, the structure of the eye of _Serolis schythei_, a species found in shallow water ranging from 4 to 70 fathoms, with the eyes of _Serolis bromleyana_, a species living in deep water ranging from 400 to 1,975 fathoms, we cannot fail to see that the latter are undergoing a process of degeneration; the retinulæ and pigment being absent, and nothing left of the complicated structure of the Isopod eye but the remnants of the crystalline cones and corneal facets (see figs. 4 and 5, p. 74). Taking the genus _Serolis_ alone, it has been said ‘that in all the shallow-water forms the eye is relatively small but very conspicuous from the abundant deposition of pigment; in all the deep-sea forms, with the exception of _S. gracilis_, where the eye seems to be disappearing, it is relatively larger but not so conspicuous, owing to the fact that little or no pigment is present.’ In many groups of animals it has been shown that some of the deep-sea species are relatively much larger in size than the shallow-water species, and that others, more rarely, are much smaller, the abysmal fauna reminding us in this respect of the characters of the alpine flora. The Isopoda show many examples of this largeness in size, thus _Bathynomus giganteus_, dredged by Professor Agassiz off the Tortugas at a depth of over 900 fathoms, reaches the enormous size, for an Isopod, of 9 inches (fig. 15). _Stenetrium haswelli_, again, is larger than any of the shallow-water species of the genus, and the same remark applies to the deep-sea species of the genus _Ichnosoma_, while _Iolanthe acanthonotus_, from a depth of nearly 2,000 fathoms, is considerably larger than most of the shallow-water Asellidæ. There is another very common character of deep-sea Crustacea that is also well exemplified in the group of the Isopods, and that is the extraordinary length and number of the spines covering the body. I have already referred to this character in the supposed deep-sea Copepod _Pontostratiotes abyssicola_, and I shall have again to refer to it in treating of the Decapoda and other groups of the Crustacea.