THE FROGS AND TOADS 



481 



THE WATER FKOG. 



The absence of the black marking behind the eye which 



distinguishes the Grass Frog should be noticed. 



line down the middle of his back, or he 

 may not. He may be bright green, or 

 dirty brown ; he may be both ; he may 

 be neither. Similarly with the toads. 

 I have found terra-cotta, greenish, blackish 

 and brownish specimens. The Natter- 



so that the frog appears dark- 

 coloured ; a high temperature 

 causes them to contract, so that 

 the frog appears light-coloured. 



Normally a frog's skin is 

 damp and a toad's skin dry, 

 though the latter becomes 

 plentifully bedewed on occasion. 

 In both cases the moisture is 

 secreted by numerous and im- 

 portant glands, and the speed 

 with which it can evaporate must 

 depend on the humidity of the 

 air, so that the latter no doubt 

 has some influence on coloration. 

 Both frogs and toads appear 

 paler in the evening, but this 

 may be due to the bodies 

 forming a bright reflecting 

 surface. 



The Grass Frog adapts him- 

 self with great nicety to his natural 

 surroundings. The fact, however, that 

 when one wishes him to adapt himself to 

 artificial surroundings he usually does not 

 (and vice versa), shows either that we do 

 not properly understand the conditions 



jack is more distinctive, for the yellow which stimulate his pigment cells, or that 

 stripe from the crown of his head down his the frog knows his own business, and pre- 

 back seems to be always present in British fers to manage it his own way. The 

 examples. His under surface is often latter alternative implies that the colour- 

 white with black markings, and his upper change is to some extent under the control 



surface one shade or another of yellow 

 brown, often with terra-cotta blotchings. 



Good reason for this infinite variety is 

 to be found in the structure of the 

 skins of tailless amphibians. 

 The Grass Frog's skin may be 

 taken as typical. In the super- 

 ficial layers of this such pigment 

 as appears is diffused and 

 yellowish in colour. On the 

 surface of the underlying por- 

 tion {derma) there are layers 

 of irregular and stellate cells 

 containing a much darker pig- 

 ment. In the living frog these 

 cells contract, or expand, in 

 response to sensory impressions 

 from without, such impressions 

 being conveyed to the frog's 

 brain partly through the eye and 

 partly througli the skin itself. 



A low temperature, for ex- 

 ample, causes them to radiate 

 pigmented branches outwards. 



of the frog. 



A further interesting feature of his skin 

 is that it forms a subsidiary breathing 

 organ. During the subaquatic hibernation 



THE COMMON TOAD. 



The large parotid glands behind the eye. the warts on the body, 



and the webbing of the hind feet, should be noticed. 



