166 



ADAPTATIONS TO ARHYTHMIC ACTIVITY 



vision has occurred, still further steps may be taken. Thus in Leptodeira 

 the rods are very numerous, long, and slender, and the bodies of the 

 cones have gotten up out of their way, their ellipsoids being perched on 

 the tips of the rods like so many pumpkins on a picket fence (Fig. 69) . 

 The cone outer segments themselves are very much larger than in 

 pure-cone diurnal forms like N. natrix, Coluber, etc. Rhodopsin is 

 abundant in Leptodeira, and the retina is so sensitive that the pupil 

 closes completely like that of a gecko. In Tarbophis and Dasypeltis the 



Fig. 68 — Transmutation in snakes, x 1000. 



a, visual-cell types of a secretive colubrid, the scarlet snake, Cemophora coccinea. Compare 

 Figure 26, p. 63. Types A and B have enlarged outer segments, but Type G (which is 

 greatly outnumbered by A + B, as in diurnal forms) is the most rod-like of the three. 



b, visual-cell types of a crotalid, the copperhead, Aghstrodon mokasen. Types A and B 

 have remained cones, but Type C (which greatly outnumbers A-i-B) is a perfea rod and 

 contains a rhodopsin. 



cones seem definitely to have lost importance, for while they are still 

 elongated far beyond the usual position, their bodies and outer segments 

 are much reduced in size as compared with Leptodeira. In these three 

 genera (as also, it happens, in the flying-squirrels) the visual cells are in 

 a condition of 'permanent dark-adaptation' (in terms of the photome- 

 chanical changes, which do not occur in snakes or squirrels) and the 

 animals are strongly nocturnal — thus really lying beyond the scope of 

 this chapter though serving to show the lengths to which a species can 

 go, if it must, to change its habits and their structural basis. 



