368 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, 



account for the presence of animals with eyes in dark caves and the 

 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 hght," 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 my prepara- 

 tions 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 

 diifering 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 XXXI. 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 and 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 Grenacher'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 m'arkedly 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 extra- 

 ordinarily unlike their nearest congeners living at the surface and in the light, 

 80 that we are forbidden to suppose that they may 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, -n-here 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 

 necera, the facetted cornea is the last part of the eye to disappear. Hence totally 

 blind animals may seem to have well- developed ejes. 



