576 HIGHER FISHES 



The Holostean Retina — The retina of Amia — except for its visual- 

 cell pattern — is quite in line with teleostean retinae histologically. The 

 lamina vitrea is particularly thick. The pigment-epithelial cells have long 

 processes reaching nearly to the limitans. These are fine and multiple, 

 so that the cell as a whole is structurally intermediate between those 

 shown in Figure 20d and 20e (p. 44). The horizontal cells form two 

 layers, those of the outer tending to be chunky parallelopipeds as in the 

 lowest fishes, but the inner ones ropy and seemingly on the way to be- 

 coming fibrous and 'conductive' in appearance like those of the higher 

 vertebrates (see p. 49). The bipolar and amacrine nuclei form the four 

 loose remaining rows of the inner nuclear layer. The outer nuclear layer 

 contains three ragged rows, the ganglion-cell layer a single scattered 

 row of nuclei. The visual cells are described in connection with those 

 of the teleosts (see Fig. 170b, p. 587). 



The retina of Lepisosteus has never been fully described. Some vague 

 statements of Mary McEwan suggest however that it is very much like 

 that of Amia. 



In both genera of holosteans the optic nerve is essentially circular in 

 cross-sectional outline, but its nervous substance is in the form of a 

 broad, thick ribbon which has been accordion-pleated edgewise to fit 

 it into a tubular sheath. Correspondingly, the optic 'disc' or nerve head 

 is not round, but vertically elongate as in many predaceous teleosts with 

 similarly high cone-to-rod ratios (see pp. 179-80). 



Teleosts — Ocular structure in the chondrostean-holostean-teleostean 

 line culminates here in a pattern whose new features are of absorbing 

 interest from the physiological and ecological standpoints, but must be 

 studied purely for their own sake since they have not been passed on to 

 any higher groups. We can make only physiological comparisons be- 

 tween the teleosts and the amphibians. The origins of all peculiarly 

 amphibian ocular features must be sought far from these 'highest' fishes, 

 in the imperfectly-known chondrostean-dipnoan-crossopterygian series 

 of patterns. 



The teleosts long ago split into two great lines : the malacopterygians 

 (soft-rayed fishes), in most of which the swim-bladder, an ancient lung, 

 remained open to the throat (hence the approximately equivalent name 

 'Physostomi' for these fishes) ; and the spiny-rayed fishes or acanthop- 

 terygians, all of which have the swim-bladder closed off, and belong to 

 the 'Physoclisti' along with a few groups — synentognaths (halfbeaks, 

 needlefishes), cyprinodonts (killifishes) , and the 'Anacanthini' (cods, 



