294 



Mormyrid 



THE EYE IN EVOLUTION 



entire eye apart from a small fibrous zone around the oiDtic nerve (the 

 soles, Soleidpe) ; sometimes it forms discontinuons islands (the elephant- 

 fish family, Mormyridse) ; sometimes it becomes partially calcified, 

 and exceptionally, as in Tetragonopterus, this transformation is com- 

 plete. Scleral ossicles formed of true bone are also usually found, 

 typically as thin plates embedded in the fibrous tissue of the sclera, 

 situated temporally and nasally anterior and external to the cartilage ; 

 occasionally in active types with large eyes these combine to form a 

 complete osseous ring of considerable strength (the sword-fish, Xiphias ; 

 tunny, Thuimiis).^ 







Fig. 325 — The Cornea of the Carp. 

 iShowing the thick epitheHuni (Smelser and Chen). 



Xiphias 



Minnow 



The cornea, usually elliptical with the long axis horizontal, 

 (Grynfeltt, 191 () : Verrier, 1927). is frequently irregular and grooved 

 and has a variable constitution. In some forms it shows the usual 

 vertebrate configuration, the substantia propria being relatively 

 homogeneous (Salmonidce — salmon, trout ; Cyprinidse — minnows and 

 carps ; Esocidae — pike) (Fig. 325) ; but in others it is uniquely complex, 

 4 layers being readily distinguishable : 



(1) A dermal layer, derived from and continuous with the skin, 

 consisting of a multi-layered, usually thick ejiithelium. Bowman's 

 membrane and the superficial portion of the substantia propria. 



^ It is to be remembered that the scleral ossicles of Sauropsida are homologous 

 not with the scleral ossicles of fishes, but with the circumorbital bones. The ossicles 

 of tli(> sturgeon are derived not from the sclera but from the skin (H. Miiller, 1872), 

 p. 317. 



