xii. 5 SKIN OF AMPHIBIA 299 



The glands of the skin are more highly developed than in fishes, 

 and are of two types, mucous and poison glands. Both of these consist 

 of little sacs of gland-cells, derived from the epidermis. The mucus 

 serves to keep the skin moist, this being essential if the skin is to 

 respire; the secretion may perhaps also serve for temperature regula- 

 tion. The problem of regulation of temperature is important for all 

 terrestial animals, since air conducts heat much less well than water 

 and therefore violent changes of temperature are met. Evaporation 

 produces large influences on temperature and no doubt it was the 

 adjustment of these effects that led to the development of temperature- 

 regulating mechanisms in birds and mammals. Frogs in dry air are 

 always found to be colder than their environment, the difference 

 being sometimes as much as 5 C. It is probable that in some cir- 

 cumstances use is made of this cooling, since tree-frogs (Hyla) may 

 be found fully exposed to tropical sunlight, which would be expected 

 to raise their temperatures to a lethal level. On the other hand, the 

 loss of water involved by evaporation in this way would presumably 

 soon become serious. 



The poison glands or granular glands are less developed in Rana 

 than in Bufo ('the envenom'd toad') where they are collected into 

 masses, the parotoid glands. The effect of the poison on man is to 

 produce an irritation of the eyes and nose; only rarely does it affect 

 the skin of the hands. When swallowed it produces nausea and has 

 a digitalis-like action on the heart. The poison of Dendrobates of 

 Colombia is used on arrows; it acts on the nervous system. 



Some amphibia have characteristic smells, produced by secretions, 

 and these are probably used to attract the sexes to each other. In 

 some male newts (Plethodontidae) there are special collections of these 

 gland-cells below the chin. 



Another use of glandular secretions is to keep the eyes and nostrils 

 free from obstruction. The demands of terrestrial life require the 

 production of numerous such special devices and lead to the com- 

 plexity that we recognize as an attribute of these 'higher' animals. 



5. Colours of Amphibia 



The use of colour is also highly developed in amphibia. The 

 animals are often greenish and the colour is produced by three layers 

 of pigment cells, melanophores lying deepest, guanophores, full of 

 granules, which by diffraction produce a blue-green colour, and yellow 

 lipophores, overlying these and filtering out the blue. Change of colour 

 is produced by expansion of the pigment in the melanophores under 



