REPORT OX THE ISOPODA. 21 



Serohs gracilis, iu wliich species the eye seems to be disappearing (see p. 24), it 

 is relatively larger but not so conspicuous, owing to the fact that little or no pigment is 

 present ; these external points of difference between the eyes of the deep-sea and shallow- 

 water species are accompanied by very striking differences in their minute structure. 

 Since, however, the shallow-water species agree more closely with other Ai-thropoda in 

 the structure of the eye, it will be more convenient to commence with them before 

 describing in greater detail the deep-sea species. 



Among the shallow-water species I have investigated the minute structure of the eye 

 in the following: — Scrolis schythei, Serolis paradoxa, Serolis latifrons, and Senilis cor- 

 nuta. In all there is a fundamental similarity, Ijut there are certain differences in detail. 



In Serolis schythei the eye (PI. IX. fig. 2) is facetted externally, and a transverse 

 section shows that each facet forms a doubly convex lens, the posterior convexity beino- 

 more marked than the anterior. Corresponding to each lens is a vitreous body (i-) of an 

 oval conical form, and composed, as in other Isopods, of two halves joined in the middle 

 line ; as a general rule the vitreous bodies have the even regular form displayed in the 

 figure, but some are uneven, one half being larger than the other; surrounding the vitreous 

 body are the remains of the two cells which gave rise to them, and above, between the 

 vitreous body and the cornea, are two nuclei (" nuclei of Semjjcr ") which belong to 

 these cells. The whole " vitrella "^ is enclosed in a sheath of deep black pigment cells. 



Below the vitrella is the retinula, which consists of only four cells ; this is an un- 

 usually small number ; from Grenacher's work upon the Arthropod eye already quoted, 

 it appears that five or seven cells is the usual number, aud in some cases the retinula 

 contains a greater number of cells ; Gammarus locusta would seem to be the only known 

 Crustacean besides Serolis in which the retinula is composed of so few as four elements. 

 Each of the retinula cells is club-shaped when viewed in front (fig. 2) ; seen laterally they 

 are hatchet-shaped (figs. 18, 19); at about the middle of the cell is an oval swelling 

 where the nucleus is situated ; the nucleus is oval iu form, the long axis corresponding 

 to the long axis of the cell, and is contained in a cavity ; in the interior of each nucleus 

 is a small highly refracting nucleolus. The retinula cells arc clothed externally -nith a 

 coating of pigment, which is more especially developed at the upper swollen extremity 

 and gradually decreases in amount towards the lower end ; this pigment apj)ears to be 

 contained in long branched connective tissue cells. The retinal cells themselves, however, 

 do contain intrinsic pigment iu addition to this adventitious sheath, as is shown by 

 transverse sections. Fig. 20 represents a series of sections through a single retinula at 

 various points which are indicated iu the description of plates ; from these sections it 

 may be seen that the retinal cells contain abundant pigment granules within their own 



^ The nomenclature used in this description is taken from Grenacher's Untersuchungen iiber das Sehorgan der 

 Arthropoden, Gcittingen, 1879; and from a Jlemoir on the Eyes of Limidus and Scorpio by Prof Lankester and 

 Mr. A. G. Bourne {Quart. Journ. Micr. Sci., N. S., vol. xxiii. p. 177, 18S3). 



