THE ELASMOBRANCH FISHES 



267 



From an external view the eve is seen to be further protected by the orbit 

 and in a way by upper and lower membranous lids. In Chlamydoselachus the 

 lower lid is a deep fold which is interesting because a part of its inner surface 

 is covered witli placoid scales. Tlie same is true of Mustelus. In fact it was from 

 the possession of scales that the lower lid was formerly incorrectly supposed 

 to be a newlv formed structure. 





C D 



Fig. 233. Sagittal section of eye. (From Franz.) A. Acanthias. B. CetorJdnus. C. Spinax. 

 D. Raja iatis. 



ah., space for aqueous humor; c, ciliary body; ch., choroid coat; en., cornea; ir., iris; 

 I., lens; l.m., lens muscle; o.p., optic pedicel; r., retina; sc, sclerotic coat; sJ., dorsal suspen- 

 sory ligament; sch., suprachoroidea ; //, optic nerve. 



The eyelids are movable in only a few Elasmobranchs. In Scyllium, while 

 they are so sluggish as rarely to close, yet the eye has been observed to bat. In 

 many other forms the lids are more or less completely devoid of musculature 

 and are therefore immovable. In others musculature may be fairly well de- 

 veloped (see p. 104, fig. 106). In the rays the eyes are firmly fixed so that 

 movement is impossible. 



Structure op Adult Eye 



The structure of the adult eye in a number of Elasmobranchs has been studied 

 in detail by Franz (1905). In a median sagittal section through the eye of 

 Acanthias striking the optic pedicel (fig. 238a) the various structures making 

 up the eye appear. In the anterior part is the clear cornea (en.) and extending 

 almost against it is the spherical crystalline lens (I.). The clear space between 

 the two (aJi.) is filled with aqueous humor. The dark layer {ir., fig. 233c) is 

 the pigmented iris, a circular curtain which contains the color of the eye; the 



