FISHES 279 



functions as an air-breathing lung. Although the C'hondrostei ai'e thus largely 

 extinct, their descendants comprise most of the modern fishes. 



The HOLO.STEI (oAo?, whole ; oareov, bone), another ancient off-shoot of 

 the primitive Chondrosteans dating from the Permian era, are represented only 

 by two extant species found in X.America, the gar-pike {Lepidosteus) and the bow- 

 fin {Amia); they are characterized by the completeness of their bony skeleton. 



The TELEOSTEi (reAeo?, complete ; oareov, bone) or modern bony fishes, 

 probably stand in a continuous genetic line with the Holosteans and include the 

 vast majority of fishes now alive — some 20,000 si^ecies. They date from Jurassic 

 times, and because of their high differentiation probably began to assume their 

 overwhelming preponderance as inhabitants of the seas in the later Cretaceous 

 and Tertiary epochs. As would be imagined they exhibit the most fully developed 

 and specialized eyes of all fishes. 



We shall first discuss in some detail the characteristics of the eyes of the 

 species at each end of the scale — the relatively simple eyes of Selachians and 

 the highly developed eyes of Teleosteans, and thereafter note the essential 

 differences in the intermediate classes. 



Chondrichthyes (Elasmobranchii) 



The Selachian Eye 



THE SELACHIANS are divided into two orders, between which, however, the 

 eyes differ little — (i) an older group of fusiform-shaped fishes, the euselachii, 

 comprising the sharks and their relative, the dogfish (Fig. 294), and (ii) the 

 BATOiDEi, modified forms with flattened bodies comprising the skate-ray- 

 torpedo group (Figs. 295 and 296). All are voracious carnivorous fishes with 

 cartilaginous skeletons, and with few exceptions, such as the fresh-water saw- 

 fish, Pristis, marine in habitat. Most of them are of benthonic habits and 

 their eyes are therefore specifically adapted for dim illumination ; occasionally 

 in abyssal forms which frequent the sea-bottom, the eyes have become vestigial 

 and blind as in the deep-sea rays, Benthobatis, Typhlonarke and Bengalichthys.^ 



The general configuration of the eye is simple with the tj^Dical 

 ellipsoidal shape and the scleral cartilage found generally in fishes 

 (Figs. 297-9). The main selachian characteristics are : 



a thick ejyichoroid on the outer surface of the choroid, somewhat 

 reminiscent of that seen in the lamprey, and icithin the choroid an u7iusuaUy 

 elaborate tapetum lucidum, a structure which {unlike the tapetum of Teleo- 

 steans) has a vi sued function in dim illuminations ; 



a ciliary zone provided with antero-posterior folds giving rise dorsally 

 to a suspensory ligament of the lens and ventrally to a cushion-like papilla 

 provided ivith an ectodermal protractor lentis mivscle ; 



a sluggishly mobile iris provided ivith primitive sphincter and 

 dilatator muscles, at this stage, hoivever, autonomously contractile and 

 without a nerve supply ; 



a shallow anterior chamber icithout an annular ligament {as in the 

 lamprey), without a pectinate ligayne^it or other structures in the free angle, 

 and without a cajial of Schlemm ; 



1 p. 724. 



Pristis 



