FISHES 313 



rotation of the eye under the transparent skin. The thin scleral 

 cartilage reaches only to the equator, and there is no amiular ligament 

 or nieshwork in the angle of the anterior chamber, no ciliary body, 

 zonule or muscles, and apparently no accommodative mechanism. 

 The choroid is extremely thin and lightly pigmented without an 

 argentea, and there is a well-developed membrana vasculosa retinae 

 which, however, can be separated from the retina only with difficulty. 

 The iris shows no evidence of pupillary musculature. 



The retina shows several peculiarities. The cells of the pigmentary 



Fig. 361. Fig. 362. 



Fig. 361. — Diagram of a Dipnoau eye. 



Ch, choroid ; CE, corneal epithelium ; IC, intermediate corneal tissue 

 MV, membrana vasculosa retina? ; OX, optic nerve ; S, scleral cartilage 

 Sc, sclera ; SC, scleral cornea. 



Fig. 362. — The pupil of Xeoceratodus. 



epithelium are enormously large so that this layer is thicker than the 

 entire choroid and they are provided with numerous long filamentary 

 processes (Fig. 363). In the visual retina the outer nuclear layer con- 

 sists of 2 rows of cells, the inner nuclear layer of 4, and there is a single 

 row of ganglion cells. The rods are unique — enormous and cone-like 

 with a large oil-droplet (except in N eoceratodus) and a paraboloid — 

 probably representing a very primitive type, derived, according to 

 Walls (1942), from an archaic early cone; in Protopterus the cones are 

 of two forms, single and double, also provided with oil-droplets confined 

 to one member of the double cone (Fig. 364) ; in A^ eoceratodus there 

 are single cones onl}- ; and in Lepidosiren the cones are absent and the 

 retina is pure-rod (with oil-droplets) (Kerr, 1902-19). 



In Profopterns the optic nerve, as is seen in Cyclostomes. is a 

 single cord with an ependymal core ; in Lepidosiren and N eoceratodus 



