268 



THE ELASMOBRANCH FISHES 



' ' -rd. 



aperture within the iris is the pupil. At (c.) is the ciliary body. The large 

 cavity back of the lens is filled with the vitreous body. The lining of this cavity 

 is the retina (r. ) under which is the pigmented choroid layer (ch.). Between 

 the choroid and the sclerotic layers there is a suprachoroidea (sch.) of con- 

 nective tissue. As an outer protective capsule, and continuing in front to the 

 cornea, is the strong sclerotic layer (.sr.) through which the optic nerve passes. 

 If the eye of Acanihias be compared with that of the closely allied Spinax 

 niger (figs. 233a and c), which is an inhabitant of the deep sea, several im- 

 portant differences will be noted. In the first place the eyeball in Spinax bulges 

 out in front, and the enlarged lens extends far into the pupil. As a result, in 

 Spinax the pupil is of immense size and is thus correlated with the environ- 

 ment in which little light is present. The 

 sclerotic layer in Spinax is thin and the 

 optic ])edicel is absent. 



Two other types, Ceforhinus maximus, 

 the basking shark (fig. 233b), and Raja 

 hatis (fig. 233d), may be briefly noted. In 

 CetorJiinus the eye is of large size and is 

 marked by several distinguishing fea- 

 tures. The pupil is small as is also the lens 

 located well back in the cavity in the vit- 

 reous body. Back of the choroid, the supra- 

 choroid coat (sch.) extends dorsally and 

 ventrally as a strongly vascular area. The 

 sclera forms an unusually heavy carti- 

 laginous capsule from which the optic 

 pedicel (o.p.) is removed by a wide mass 

 of connective tissue. 



In Raja (fig. 233d) the eye is of peculiar 

 shape. The cornea bulges forward at the 

 ventral margin, and the lens sinks deep 

 within the eye. The pupil here is of minute size and the layers of the anterior 

 part of the eye are exceedingly thin. On the posterior boimdary of the eye, 

 however, the sclera is heawv^ and is separated from the choroid by a highly 

 vascular suprachoroidea. The optic pedicel (o.p.) is wide and is joined to the 

 sclera by connective tissue. 



Fig. 234. Section through the retina 

 of the eye, Mustclus vulgaris. (From 

 Schaper.) 



CO., cone; ei)., epithelial lining; 

 i.h., inner wide layer; i.p., inner 

 plexus layer; o./i., outer heavy layer ; 

 rd., rod. 



FINER STRUCTURE OF RETINA 



A section through the retina of the eye of Mustelus (fig. 234) (Schaper, 1899) 

 shows that this important coat of the eye is made up of numerous strata of 

 cells and fibers. These may be briefly described from the inner to the outer side 

 of the retina as an inner ganglionic layer containing fibers and cells, then an 

 inner plexus layer (i.p.). Following this is an inner wide layer (i.h.) and an 

 outer narrow heavy layer (o.h.). There then follows the important area of 



