FISHES 



307 



40% more rods than the doi.sal area (]M(jore et al., 1950), iu tlie pelagic dragonet, 

 Callionymus, the dorsal half is almost entirely populated by cones, the ventral 

 by rods (Vilter, 1947), while in the sardine, Clupea pUchardus, which feeds on 

 Crvistaceans in the water above it, this relationship is reversed (Vilter, 1950). 

 This adaptation may develop with the growth of the fish and a change in its 

 habitat ; thus in the elver (and cavernicolous eels) the rods are more numerous 

 in the ventral part of the retina, while in adult eels in rivers they are more 

 numerous in the dorsal area, (Vilter, 1951). 



The rods are usually small, elongated and very numerous, although 

 in some species (the cat-fish, Ameiurus) they are thick, plump and few in 

 number (18,400/sq. mm.). The cones, in contradistinction to the rods, 



Ameiurus 



Figs. 347 to 349. 



-The Visual Cells of Teleosts (x 1,000) 

 (Gordon Walls). 



i 



4- 



! 



Fig. 341 



Fig. 348. 



Fig. 347. — The cone and rod of the goldfish, Carassius. 



Fig. 348. — A single cone, a twin cone and a rod of the pike-perch, 

 Stizosiedion. 



Fig. 349. — The twin cone of the sunfish, Lepomis (light-adapted) and 

 the conjugate element of Fundulus (after Butcher, 1938). 



c, "clear mass" and g, "granular mass" in the conjugate element; 

 e, ellipsoid ; /, footpiece ; /, external limiting membrane ; //(, myoid ; 

 n, nucleus ; o, outer .segment. 



