AMPHIBIOUS VISION IN TELEOSTS 



433 



maximally. When it goes under water the accommodated eye naturally 

 becomes strongly hypermetropic. Whether perfect emmetropia can be 

 restored in water by complete relaxation of the retractor lentis is un- 

 known, and unlikely. The great increase in the brightness of the retinal 

 image in air is reflected in a predominance of cones, and their distri- 

 bution is clearly adaptive to the downward incidence of the sunlight. A 

 substantial portion of the inferior half of the retina is pure-cone, the 

 remainder duplex (with about 80% cones) except for a narrow pure-rod 

 zone in the extreme superior periphery. The pigment epithelium is excep- 

 tionally thick, and rich in pigment. The rich cone population frees the 

 animal from dazzlement, and makes possible a visual acuity adequate to 

 the pursuit of its active food (largely insects) in a quite lizard-like 

 fashion, the fish skipping about upon its stiff pectoral fins. By compari- 



Fig. 146 — Dialommus juscus, an amphibious blenny. Based upon figures of Breder and Gresser. 



a, anterior end. b, schematic front view of eye. c, schematic horizontal section of eyeball. 



c- cornea; i- iris; /- lens; w, w- unpigmented 'windows' in cornea. 



son with even so sharp-sighted a predator as the pike, Periophthalmus 

 shows to advantage; for it has been found to have about 225,000 visual 

 cells and 90,000 ganglion cells per square millimeter of retina, while 

 counts in Esox have shown 50,600 rods, 5600 cones, and 3512 ganglion 

 cells per square millimeter. 



One of the surf-loving rock blennies, Dialommus juscus, has been 

 recently studied, though the investigators had to give up when they tried 

 to interpret the eye. At first glance, Dialommus appears to have two 

 pupils, fore and aft (Fig. 146). There is actually but one aperture in the 

 iris itself, the two clear areas being in the cornea (Fig. 146c) which is 

 otherwise heavily pigmented — a great exaggeration of the eyeshade-like 

 dark pigmentation of the upper part of the cornea in some of the needle- 

 fishes and in Torpedo. Nothing is known of the refraction and accommo- 

 dation of Dialommus, but it seems to have made an ingenious adjustment 



