420 LIFE : OUTLINES OF GENERAL BIOLOGY 



injurious effects of light, to which it is extraordinarily susceptible. 

 This leads our thoughts to the origin of the earthworm's sub- 

 terranean and nocturnal habits. 



COLOUR CHANGE.- When a frog, normally of a greenish hue, is 

 put into a dark moist box in a cold room, it becomes dusky in a 

 few hours, and blackish in a couple of days. But if it be put in 

 a whitened box in a warmish, well-lighted room it puts on a lemon 

 colour. The darkening is due to the "expansion" of branched black 

 pigment-cells (melanophores) in the epidermis and dermis. The 

 op|X)site change is due to the "contraction" of the black pigment- 

 cells to microscopic pin-points, and this brings into prominence 

 certain yellowish pigment-cells (xanthophores) in the dermis. It 

 is not certain whether a melanophore in a frog draws itself together 



iG. 52. 



Complex Chromatophorc from a Prawn {Praunus flexitosus). After Degncr. 

 The pigment Hows out along the complex branches or contracts centri- 

 j>etally. This unusually complicated chromatophorc appears to arise 

 from a combination or syncytium of cells. 



as a whole, just as if it were an Amoeba, or whether the "contraction" 

 means that the pigment granules flock towards the centre, as is 

 certainly the case in crustaceans like the prawn. It used to be 

 thought that the contraction or expansion of the pigment-cells in 

 the frog was controlled by fine branches of the sympathetic nervous 

 .system; but the brilliant researches of Hogben and Winton have 

 proved that the control is due to the hormone secreted by the 

 pituitary lx)dy in fluctuating (juantities. In reptiles, like the chame- 

 leon and the so-called "horned toad", there are superficial, yellowish, 

 "interference" cells or xanthophores, and deeper melanophores with 

 branches passing up into the outer layer. Most of the colour change 

 is due to the up-and-down movement of the pigment granules in 

 the melanophores. and this is mainly under the control of the 

 adrenalm hormone secreted by the suprarenal bodies. In fishes, 

 such as the plaice and the flounder— where the colour change is 

 very rapid, the creature sometimes making itself invisible almost 

 instantaneously— the control is effected by branches from the 



