Besides its enormous size _Bathynomus_ possesses some other characters that may be correlated with its deep-sea environment. The respiratory organs are quite different from those of other Isopods; instead of being borne by the abdominal appendages, they are in the form of branched outgrowths from the body-wall containing numerous blood-lacunæ, and the appendages simply act as opercula to cover and protect them. The eyes of the Bathynomus too are remarkably well developed, each one bearing 4,000 facets, and they are directed not dorsally as in the Cymothoadæ, but ventrally. The cause of these curious modifications of structure in Bathynomus is by no means clear, but it is quite probable that they are connected with the conditions of pressure and light in the deep sea. It is a remarkable fact that the other deep-sea Isopods do not exhibit precisely these modifications, and it might be supposed that the same causes would produce the same or similar effects on the structure of animals belonging to the same order. That is perfectly true, but we cannot yet determine how long ago any one species has taken to a deep-sea life, or what length of time, in other words, these conditions have been at work in modifying the structure of the organism. A recent immigrant into the abyss will naturally exhibit closer affinities with its shallow-water allies than those that have dwelt in the region since secondary or tertiary times. If we take this into consideration we should expect to find considerable differences occurring between deep-sea species of the same order, which is precisely what we do find. [Illustration: FIG. 15.—_Bathynomus giganteus._ From a depth of 1,740 metres. (From Filhol.)] Concerning the Cirripedia, that curious group of profoundly modified Crustacea that includes the barnacles and acorn shells, Dr. Hoek writes in the ‘Challenger’ monograph:— ‘Though unquestionably by far the greater part of the known Cirripedia are shallow-water species, and though some of the species are capable of living at a considerable variety of depths, as, for instance, _Scalpellum stroemii_, yet it must be granted that the number of true deep-sea species of Cirripedia is very considerable.’ Only two genera, however, occur in depths of over 1,000 fathoms, and these—_Scalpellum_ and _Verruca_—occur also as fossils in secondary and tertiary deposits. The oldest of all fossil cirripedes, however, namely, _Pollicipes_, never occurs, at the present day, in deep water, but is purely littoral or neritic in habit. But what is perhaps more interesting still is the fact, that, when we come to compare the living and the fossil species, we find that in the one genus (_Scalpellum_) the deep-sea forms have preserved the more archaic characters, and in the other (_Pollicipes_) the shallow-water forms.