CH. VIIl] FROGS, TOADS, AND NEWTS 247 



to minimise or display to utmost advantage any particular 

 pigment. The action is almost certainly a reflex depending 

 upon sensory impulses received by the retina of the eye. 

 It is this power which enables a frog to imitate with very 

 fair success the general colour effect of the varied sur- 

 roundings among which it passes its life. As it squats on 

 the ground among the grass it may well escape notice 

 until it jumps. In this position the value of the pale 

 ventral edges of the flanks becomes evident in lessening 

 the effect of the shadow cast by the body of the animal, 

 and thus causing it to appear not to project above the 

 surface of the ground. 



The surface of the skin is smooth and moist. The 

 outer horny layer is, except on the back and on the under 

 side of the toes, very thin. This fact may be realised at 

 the periodic casting or "sloughing" of the horny layer, when 

 it appears as a translucent pellicle which the frog, econo- 

 mically, swallows. Beneath the surface layers are numerous 

 goblet- or mucous-cells, which are stated by Pfitzner 1 to 

 secrete a substance which separates the upper layer from 

 that below, and brings about a complete shedding of the 

 skin. Two kinds of glands are present in the skin, serous 

 and mucous : the former are found chiefly upon the back 

 and correspond to the poison-glands of the skin of toads ; 

 the latter are more numerous and more evenly distributed 

 over the body, their protoplasm swells up freely on the 

 addition of water. The ducts of both kinds of gland open 

 upon the surface, and play an important part in keeping 

 the skin moist and permeable by the gases of respiration. 



1 Morph. Jahrb. vi. 



