26 THE VOYAGE OF H.M.S. CHALLENGEE. 



vitreous body itself, instead of being placed above it and in close proximity to the 

 cornea. 



1 was unable to detect with certainty these nuclei of Semper in the eye of Serolis 

 hromh'i/ana, but in another deep-sea form, Serolis necBra, having an eye of similar 

 structure they were very conspicuous, and situated above the "vitreous body" between 

 it and the cornea. Each of the vitreous bodies of Serolis hromleyana is enclosed in a 

 cup-shaped sheath of tissue (PI. X. fig. 5) in which no trace whatever of any structure could 

 be made out. This sheath possibly corresponds to the j)igment sheath which encloses the 

 vitreous body in other Arthropoda, but in Seivlis hromleyana it is entirely free from 

 pigment. Below this portion of the eye, and corresponding to each of the vitreous 

 lenses, is a roundish mass of cells which are separated into grou23s by ramifying trabeculse 

 of pigmented connective tissue ; from the posterior end of this mass of cells a delicate 

 ];)undle of nerve fibres arises which passes back to the ganglion ; it is clothed externally 

 by a layer of pigmentiferous ramified cells continuous with those in front. 



The eyes of Serolis necera resemble in external characters the eyes in the shallow- 

 water species of Serolis; they are distinctly faceted, and a great deal of pigment is present ; 

 they are also proportionately larger than in any of the shallow- water species. 



The minute structure, however (fig. 3), agrees in the main with that of Serolis hrom- 

 leyana, but is a little less degenerated ; the cornea, as ah-eady stated, is distinctly faceted ; 

 beneath each facet are two large and conspicuous nuclei ("nuclei of Semper"), these are 

 round or oval, sometimes pear-shaped ; they are surrounded by a distinct membrane 

 showing a double contour, and contain a highly refractive nucleolus. The vitreous 

 bodies, like those of Serolis hromleyana, are of an irregular oval form and granular opaque 

 consistency ; the upper half is encircled by a ring of pigment. At the lower end of each 

 is a roundish mass of small nucleated cells [g), probably nerve cells, and these are con- 

 nected posteriorly with a nerve bundle, partly sheathed with pigment cells, which extends 

 into the mass of nerve cells dividing it up into lobules. Fig. 8 is a single element isolated 

 by teasing in glycerin ; the vitreous body (V) is very distinctly cup-shaped. 



The results of my investigations into the minute structure of the eye in Serolis may 

 be briefly summed up as follows : — (1) The shallow-water s^^ecies invariably possess well- 

 developed eyes which are fundamentally similar to those of other Isopoda, but differ 

 in several particulars ; the retiuulge are composed of only four cells ; the rhabdom is 

 often a highly complicated structure unlike that of other Crustacea. Another element 

 unrepresented (?) in the eyes of other Crustacea is present, consisting of two large 

 hyaline nucleated cells placed below the rhabdom and between the retinal cells. (2) 

 In the deep-sea species the eyes are either altogether absent (Serolis antarctica), 

 or, if present, show signs of structural degeneration ; no retinula, at least nothing 

 comparable to the retinula in the eyes of the shallow-water species, is present, but the 

 vitreous body is represented. The vitreous bodies may be enclosed in a sheath of 



