22 COMPARATIVE ANATOMY 



not only in relative size, but also in the structure of their cells 

 and in function. Mucus-glands, and much larger poison-glands 

 (the secretion of which is granular), can usually be distinguished, 

 the latter serving as a passive means of defence ; but intermediate 

 forms may be recognised. Smooth muscle-cells are very numerous 

 in the derm, certain of them surrounding these glands, and form- 

 ing constrictors and dilators. 



In the Anura, the blood-vessels are not always confined to the 

 derm, but in connection with the respiratory function of the skin 

 extend far into the epiderm before metamorphosis, during which 

 process the capillary loops in the epiderm increase markedly, 

 decreasing again subsequently. This may be explained by the 

 fact that during metamorphosis the gills are no longer functional 



Superficial and deep layer 

 Mucus glands. Pigment. Sensory organs. of epiderm. 



: ! 



Muscles. Derm. 



FIG. 16. SECTION THROUGH THE SKIN OF A Triton alpestris IN THE BREEDING 



SEASON. 



and pulmonary respiration alone is apparently insufficient ; an 

 additional vicarious arrangement is therefore temporarily neces- 

 sary. 



The coloration of the skin, which may undergo change, is due 

 to chromatophores of different tints in the derm. The derm is 

 similar to that of Fishes, and is, moreover, characterised by an 

 abundance of blood-vessels and nerves, as well as of smooth 

 muscle-fibres. Calcifications and even ossifications (e.g. in Cera- 

 tophrys dorsata) may occur in the derm, and in the Gymnophiona 

 definite dermal scales are present. 



A stratified stratum corneum becomes developed even in 

 perennibranchiate types, and may become more pronounced in 

 adaptation to a terrestrial existence in caducibranchiate forms at 

 metamorphosis. The cornification is especially marked along the 

 back, and may result in the formation of warts and papillae ; 

 occasionally claw-like structures are developed on the digits 

 (Xenopus, Onychodactylus). The horny layer of the epiderm is 

 shed periodically, either in pieces or entire. 



