308 



THE EYE IN EVOLUTION 



Fundnlus 



are relatively bulky (Fig. 347). These are remarkable for the presence 

 of twin and double cones, double cones, seen also in Holosteans and 

 widely distributed among most Vertebrates, occur in many Teleosteans 

 such as the roach, Rutilus (Greeff, 1899), the goldfish (Walls, 1942), 

 some of the Salmonidse (Verrier, 1935 ; McEwan, 1938), the kilhfish, 

 Fundulus, and others. They were first described by Hannover (1840), 

 M. Schultze (1867) and Dobrowolsky (1871) and consist of the fusion 

 of two dissimilar cones in the lower myoid region, one, a large cone, 

 being the chief element and the only one which participates in photo- 

 mechanical movements, the other, a smaller accessory element with an 

 unusually large paraboloid. There are two nuclei, and the two foot- 

 pieces may connect with different bipolar cells, twin cones, on the 





I ♦ %^ * If 



A«> 





Fig. 3.50. — Triple and Quadruple Cones in a Teleostean Fish. 

 A tangential section through the retina of the minnow, Phoxinus Icpvis, 

 to show double, d, triple, t, and quadruple, q, cones ( X 500) (A. H. Lyall). 



Salmo trutta 



other hand, are found only in the teleostean retina in which their 

 occurrence is widespread (Fig. 348-9). In these the two elements, 

 fused throughout their entire inner segments, are identical and both 

 contract and elongate in photomechanical movements. Twin cones 

 are more numerous in the central retina than the peripheral and in 

 surface fish than deep-sea types ; in some particularly active species 

 they are the only cone elements encountered (flat-fishes ; some species 

 of scorpion-fish, Scorpcena ; cod, Gadus ; etc.) (Wunder, 1925-30). 

 While they are thus associated with vision in bright light, they do not 

 seem to subserve accuracy of vision since they are absent from the 

 fovea when this is present. 



MULTIPLE CONES (triple and quadruple) have been described by Lyall 

 (1956-57) — triple cones ^ in the retina of the trout, Salmo trutta, which appear to 

 be anomalous double cones ; and triple and quadruple cones in the retina of the 

 minnow, Phoxinus, where they occur in considerable numbers (Fig. 350). In 



' Triple cones have also been described in the frog, Bana temporaria (p. 342) 

 an<l Me gecko, Aristelliger (p. 364). See, however, p. 253. 



