24 THE VOYAGE OF H.M.S. CHALLENGER. 



of the two hyaline cells ; the median portion is usually formed by a sinuous rod of vary- 

 ino- lenfTth which ramifies at its lower extremity into four coils; the whole structure 

 reminds one very forcibly of the Malpighian bodies of the kidney ; m some cases the 

 median rod is double, and more rarely (fig. 13) it is represented by four or five pieces; 

 it is possible that these varieties correspond to stages in growth, the more complicated 

 {e.g., figs. 11-13) being the older. 



The "hyaline cells," as already mentioned, are two in number in each retinula ; the 

 nucleus is situated close to the lower extremity ; in sections which have been slightly 

 depigmented the whole of these hyaline cells becomes tinged of a light mahogany brown, 

 Avith the exception of the nucleus, which remains unstained, and is therefore exceedingly 

 conspicuous ; in, e.g., figs. 16, 17, which are drawn from such sections, it may be seen that 

 these cells enclose almost the whole of the rhabdom, and this would seem to suggest that 

 they are concerned in its production and may therefore perhaps be modified retinula cells ; 

 the cells themselves are quite homogeneous and transparent — apart, of course, from the 

 nucleus — and in section appear exactly similar to the vitreous body ; they are, however, 

 more transparent and quite colourless, whereas the vitreous body is always of a chitin-yellow 

 colour. In a teased preparation of the eye of Serolis cornuta the addition of strong 

 nitric acid dissolves out the pigment, which is at first absorbed by the hyaline cells, 

 causing them to assume a very remarkable appearance, represented in PI. X. fig. 1 ; these 

 cells become quite granular with a crenated outer margin and of a dark greyish-black 

 colour ; when the process of depigmentation has gone on a little further the pigment is 

 dissolved out of the hyaline cells, leaving them perfectly tran.sparent and smooth, or at 

 most with a slight mahogany brown tinge. 



Among the deep-sea species of Serolis the eye is entirely absent in Serolis antarctica ; 

 its place, however, is occupied by a small tubercle not more than 1 mm. in extent, on 

 either side of the cephalic shield ; the posterior border of the latter in consequence is 

 more regular, and the postero-lateral projections which correspond to the eyes are but 

 slightly marked ; the two tubercles have the same microscopic structure as the rest of 

 the chitinous integument, and show no traces of facets ; their interior is filled with a plug 

 of connective tissue, in which no vitreous bodies or retinal elements could be detected. 

 In Serolis gracilis, another deep-sea form, the eyes present some curious peculiarities. 

 In two specimens the eyes are well developed, though small, and resemble entirely 

 the eyes of Serolis hromleyana ; they are conspicuous from their whitish colour, 

 which contrasts with the surrounding integument ; in one specimen the ocular protu- 

 berance is only partly occupied by the optic tissues, the rest being exactly similar in 

 colour to the general integument of the body ; in another sjjecimen there is no trace 

 whatever of any optic structures, though the oval-shaped elevations of the eye are present 

 l)ut dark bluish in colour like the rest of the integument. In this species, therefore, the 

 eye seems to be just on the verge of disappearance. 



