312 THE EYE IN EVOLUTION 



fused lids become extremely thin and transparent forming a " secondary 

 spectacle ",^ and between them and the corneal epithelium there is a 

 closed " conjunctival sac " lined by epithelium (Fig. 279). 



The Salmonidae (the salmon-trout family) have a peculiar arrangement of 

 lids. The posterior lid is of the usual type but the anterior, which has been 

 called a false nictitating membrane, is not derived from the skin of the circum- 

 ocular sulcus but is represented by a broad triangular fold arising deeply from 

 the anterior rim of the membranous orbit. 



The orbit is bony and completely enclosed ; its roomy cavity is 

 filled with loose tissue and venous sinuses serving as a cushion for the 

 globe, which is sometimes anchored by a tenacular ligament. The 

 extra-ocular muscles correspond with those of Selachians and are 

 carried through canals in the orbital bones where they find insertion, 

 an anterior canal serving the obliqiies, a posterior the recti (Corning, 

 1900 ; Allis, 1922) (Fig. 293). 



THE DIPNOAN EYE 



THE DIPNOI (lung- OR MUD-FISHES) are a very primitive stock with three 

 surviving representatives — Protopterus, the African lung-fish which bvirrows in 

 the earth in the dry season, the eel-like Lepidosiren from the swamjDS of the 

 Amazon, and the 6-foot long Neoceratodus from Queensland (Figs. 358-360). 



Figs. 358 to 360. — Extant Dipnoan Fish. 



Fig. 358. — Protopterus. 



Fig. 359. — Lepidosiren. 



Fig. 360. — Neoceratodus. 



The eyes of the first species have received some perfunctory study 

 which has shown them to be very primitive structures indeed (Hosch, 

 1904 ; Grynfeltt, 1911) : those of the second have been described 

 by Eochon-Duvigneaud (1943) (Fig. 361). As in the Cyclostomes, 

 there is a dermal cornea separate from the scleral cornea, allowing free 



1 p. 266. 



