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 ' ; these are Arcturus spi- 

 nosiis, A. anna, A. cornulus. 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 re- 

 markable, however, that in the other deep-sea species which I have 

 examined, viz. Arciurus brutineus and A. glacialis, and some specimens 

 of A. spinosus, the lens should show a reduction is size and an alter- 

 ation 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 retiuula-cells appear to be best developed in A. furcatus, where, 

 as shown in my drawing (fig. 8), the nucleus is placed high up, not 

 far from the commencement of the rhabdom. 1 his may also be the 

 case with A. americanus, butmy sketches are untortunatelj' not con- 

 clusive 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, «) 

 below the extremity of the rhabdom. It is possible that this re- 

 duction 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 

 oflFered 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, par- 

 ticularly 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 indi- 

 cation 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 

 which 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 sec- 

 tions of A. spinosus the amount of pigment covering the rhabdom is 

 seen to be not great (cf. Plate XXXI. figs. 10 and 5). On the other 

 ' /. e. occurring at depths greater than oOO fatborns. 



