Chapter 14 



HIGHER FISHES 



(A) Elasmobranchs 



See also pages: 



1 18 optic vesicle 



135-6 Fig. 60, taxonomy, anatomy 



150 photomechanical changes 



155 lids 



157-9,219-22,224,256 pupils 



184-7, 243, 245 area centralis 



200 habits 



216 visual cells 



225 Fig. 91 



240, 243-5 tapetum lucidum 



251, 260, 272-3, Fig. 99, accommodation 



255 ramp retina in rays 



262 protraaor lentis, ciliary folds 



264 optics 



265 persistent embryonic fissure 

 266-7 comparison with amphibians 

 372 zonule 

 comparison with snakes 

 hammerhead visual field 



268, 

 282 

 291 

 303 

 338 

 372 

 380 



eye movements 



median eyes 



water-balance 



ellipsoidality of eyeball 

 384-6 adaptation to the bottom 

 392, 394, 397-8, 402 deep-sea spp. 

 415-6 thickness of sclera 

 428-9 lids, other peculiarities 

 518-9 possibility of color vision 

 537 dermal color changes 



Most families of sharks and rays are tropical or subtropical, with 

 pelagic or benthic habits. The permanent residents of the temperate 

 zones are mostly bottom-living forms. A few species of both sharks and 

 rays live in fresh water. The chimeras are all deep-sea, bottom fishes. 

 The ocular specializations of elasmobranchs are in the direction of dim- 

 light activity, and most species are nocturnal. 



The Eye as a Whole — Elasmobranch eyes are large relative to the 

 body — largest of all (and with the largest lenses in proportion) in the 

 chimxras and such deep-sea sharks as Etmopterus, relatively small in 

 the partly-skyward-looking rays, smallest of all (except for an enormously 

 overgrown scleral cartilage) in such blind deep-sea rays as Benthobatis. 

 Mobile upper and lower lids, sometimes also a nasoventral 'nictitans', 

 are usually developed to a greater or lesser degree in sharks (Galeorhin- 

 idae, especially), though without any obvious value to the animal (see 

 Fig. 131b, p. 382). In forms whose nictitans is very active, the lower lid 

 (of which the nictitans is really a continuation) is motionless. The nicti- 

 tans alone is present, together with a circular, motionless lid-fold, in the 

 hammerheads (genus Sphyrna, —^ygcena). 



563 



