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Show 372 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, It is noteworthy that all the shallow-water species, viz. Arcturus furcatus, A. americanus, A. studeri, have lenses which are perfectly clear and transparent, and are characteristically pear-shaped. On the other hand, all those species which have an apparently partly opaque lens are deep-water forms 1; these are Arcturus spinosus, A. anna, A. cornutus. This list is not exhaustive of the deep-sea forms which I have been able to examine ; but there are no others in which the lens appears to be getting opaque. It is remarkable, however, that in the other deep-sea species which I have examined, viz. Arcturus brunneus and A. glacialis, and some specimens of A. spinosus, the lens should show a reduction is size and an alteration in shape which must impair its perfection as an organ for the passage of rays of light, if the form best suited for that purpose be that exhibited by A. furcatus. The retinula-cells appear to be best developed in A. furcatus, where, as shown in m y drawing (fig. 8), the nucleus is placed high up, not far from the commencement of the rhabdom. This may also be the case with A. americanus, but m y sketches are unfortunately not conclusive as to this point and the preparations have been since spoiled. In all the other species of Arcturus examined by me, the retinula-cells are relatively small, and the nuclei are situated (e.g. fig. 13, n) below the extremity of the rhabdom. It is possible that this reduction of the retinula-cells (which I believe with Grenacher and others to be the essential visual cells) is correlated with a commencing degeneration of the eye. If it were not for the single exception offered by A. studeri (a shallow-water species from Kerguelen), I should be disposed to lay considerable weight upon this series of facts. As it is, it does not appear to me to be safe to make any such assertion in at all a positive way. The rhabdom does seem in several of the deep-sea species, particularly in A. spinosus, to be undergoing degeneration. This is shown by its less perfect transparency and by its irregular form, and perhaps also by its very large size. It may not perhaps seem very reasonable to adduce increase of bulk in an organ as indication of degeneration. If we are to regard the rhabdom as formed by the retinula-cells, the large size of the former may be connected with the diminished size of the latter; it may therefore be a sort of degeneration. On the Lamarckian view of evolution, the increase in size of the media for concentrating the light might seem to be an attempt to keep up with the diminishing supply of light. I myself should be disposed to regard this phenomenon as a kind of " running to seed " of the non-essential part of the eye. Another point of very considerable importance in relation to the supposed degeneration of the eye is the smaller amount of pigment wbich occurs in the eyes of most of the deep-sea species examined by me. In teased preparations the rhabdom was always perfectly distinct, the yellowish-brown colour being quite visible; and in sections of A. spinosus the amount of pigment covering the rhabdom is seen to be not great (cf. Plate X X X I . figs. 10 and 5). On the other 1 /. e. occurring at depths greater than 500 fathoms. |