538 ADAPTATIONS TO PHOTIC QUALITY 



found that in lampreys (Lampetra) , the dermal changes are mediated 

 through the pituitary and not through spinal nerves, and that the median 

 eyes (see p. 339) share in the control of the changes, along with the 

 lateral eyes. 



Lampreys, Young found, become paler at night, darkening in the 

 daytime — and keep up these changes for many days when kept in con- 

 stant darkness. Similar intrinsically rhythmic changes were reported in 

 1926, by Pauli, for a teleost {Phoxinus) , where they were very slight, 

 and for larvae of Salamandra maculosa, in which they persisted for about 

 a week. Slome and Hogben, in 1929, reported marked rhythmical 

 changes in an anuran (Xenopus Icevis) kept in the dark. 



Such diurnal rhythms, which occur also in the retinal pigment cells of 

 some fishes (perhaps also in frogs) , are inherent, and outside of the con- 

 trol of the coloration by the photic stimulation of the eyes. Whatever 

 their cause, it is suppressed by light. No vertebrate exhibits any rhythm- 

 ical dermal changes when kept illuminated night and day. 

 Color Changes in Reptiles — Despite the reputation of the chameleon 

 for being able to match any colored background (and its alleged tend- 

 ency to suicide when placed upon plaids) , it can be asserted that no rep- 

 tile dynamically adapts its skin primarily to the background. The dermal 

 response to the character of the light entering the eye, or to bright light 

 in bright surroundings, may suit the animal's pattern better to the back- 

 ground; but any such improvements of concealment are even more com- 

 pletely fortuitous than they are in amphibians. 



It is only in lizards that conspicuous changes occur. Even among the 

 lizards there are only a few families which show chromatophoral changes 

 well — notably the Agamidas of the Old World and their counterparts, 

 the Iguanidae, in the Western Hemisphere; and of course the Chame- 

 leontidae. The only changeable pigment cells are the melanophores, which 

 in most reptiles underlie the iridocytes but, in poikilochromic forms, send 

 the pigment up into cell-branches which are intertwined with the vari- 

 colored iridocytes. 



To no lizard has the color of the background any significance. A 

 response to hue is made in a curiously indirect way, however, by Anolis 

 carolinemis, the 'Florida chameleon' (which is really an iguanid). Re- 

 sponses to light and darkness by paling and darkening are about as in 

 the teleosts, but they are even more at the mercy of temperature changes 

 than in amphibians. Each individual lizard has a light phase and a dark 

 phase. Wherever a species seems to have a great variety of costumes (as 



