292 



THE EYE IN EVOLUTION 



Caraasius 



the anterior chamber between the cornea and the iris, and a tensor 

 choroidece muscle ; 



the presence of a choroidal gland in most species ; 



a failure in closure of the foetal fissure allowing the protrusion of the 

 choroid through the retina as the falciform process [or alternatively the 

 emergence of a hyaloid system of vessels) to nourish the inner layers of 

 the retina, which with one known exception {the eel) is avascular ; 



an ectoderynal retractor lentis muscle at the distal end of the falciform 

 process ; 



the frequent presence of a choroidal tapetum (argentea) usually of the 

 lucidum type, but sometimes cellular, neither type, however, having a 

 visual function since they are masked by the pigment epithelium of the 

 retina ; 



a pupil usually immobile and often so large as to leave an aphakic 

 aperture ; 



a highly organized retina typically containing both rods and cones as 

 well as double cones, and sometimes a fovea. 



THE GENERAL SHAPE OF THE TELEOSTEAN EYE USUally COnformS 



to the standard type characteristic of Fishes ; in most species it is an 

 anteriorly flattened elHpsoid with the antero -posterior diameter shorter 

 than the transverse, although in slow-swimming and small-eyed types 

 the shape tends to be more nearly spherical. 



An exception to this occurs in certain deep-sea Teleosts. In these dark 

 regions the poverty of the illumination requires an immensely large lens, to 

 accommodate which the globe may acquire a tubular shape. ^ Other benthonic 

 Teleosts, giving up the struggle to make use of light in their dark environment, 

 have vestigial eyes, often covered with opaque skin ^ — one deep-sea Teleost (the 

 only known Vertebrate in such a case)has no eyes {Ipnops); as an accessory, certain 

 benthonic fishes have developed luminous organs, sometimes in association with 

 their eyes, with which they make contact with their kind.^ 



The sclera is a fibrous tunic sometimes tenuous and thin (as in the 

 goldfish, Carassius auratus), sometimes immensely thick (tjie star-gazer, 



1^ Astroscopus), reinforced by hyaline cartilage which sometimes becomes 



partly ossified (Yatabe, 1932 ; Rochon-Duvigneaud, L943 ; Woelfflin, 

 1955) : only in a few forms is cartilage lacking (some eels, Gymnotidae ; 

 the pearl-fish, Encheliophis) . Instead of forming a complete cup as in 

 Selachians, however, the cartilage is lacking in the posterior part ; the 

 general arrangement is therefore the opposite to that which occurs in 

 Birds in which the posterior segment of the sclera is reinforced by 

 cartilage (Fig. 327).* Its extent varies considerably ; sometimes it is 

 confined to a relatively narrow ring around the limbus (the salmon- 

 trout family, Salmonidae) or the equator ; sometimes it clothes the 



1 p. 332. 

 ■ p. 722. 



3 p. 736. 



^ p. 403, Fig. 496. 



