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Show 368 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, account for the presence of animals with eyes in dark caves and deep-sea, but rejected. It is accepted, however, by Henderson. This being the case it is unnecessary to make any further use of the ingenious "theory of abyssal light," and it is impossible to build up any theories with regard to the brilliant coloration of deep-sea animals. These colours must be absolutely without any secondary meaning, as must also the frequent phosphorescence of Alcyonarians and other animals living in great depths. If there were no intermediate stages between Crustacea and other animals of the deep sea with well-developed eyes and those without any trace of eyes at all, such theories might be put forward with some plausibility. It might be urged that the eyeless forms were simply peculiar in this respect ; that is to say, that just as among shallow-water genera, and even surface forms, eyes may be absent and characterize a particular genus or species by their absence, such might also be the case with genera inhabiting the deeper waters of the oceans. The numerous stages of degeneration appear to me to render this view untenable. I shall now proceed to describe, in as much detail as m y preparations allow of, the minute structure of the eye in a number of species of Arcturus. (1) Arcturus furcatus, Studer. The eye of this species is quite a typical Isopodan eye, though differing in certain details from any type that has been hitherto studied. The vitreous body is rounded conical in form and is distinctly made up of two halves. As is illustrated (Plate X X X I . fig. 4), there appear to be four nuclei corresponding to each vitreous body and lying above it. These are, I imagine, the nuclei of Semper aud the nuclei of the corneagen cells. The retinula of each eyelet is made up of six cells, which is not a number that has been hitherto met with among the Isopods. In insects this number appears to be common according to Grenadier's figures (5). The rhabdom secreted by these retinula-cells is in certain respects rather remarkable. It is conspicuous on account of its size ; it has the clear amber-yellow colour of the vitreous body ; peripherally (see Plate XXXI. figs. 5, 14-16) the rhabdom is markedly a very densely pigmented band. Towards its upper extremity the rhabdom is, as shown by puzzling, because the chief part of such deep-sea animals as can see are extraordinarily unlike their nearest congeners living at the surface and in the light, so that w e are forbidden to suppose that they m a y be species that have only lately migrated from the surface to great depths." It is unnecessary to point out that this statement does not allow for such cases as I refer to, where the eyes, although apparently like those of others, are really in various stages of degeneration. There are no doubt plenty of species in which, as in Serolis near a, the facetted cornea is the last part of the eye to disappear. Hence totally blind animals m a y seem to have well-developed eyes. |
