220 AMPHIBIA 



be deficient in oxygen, and it is possible that both the streams 

 and the quiet water become much reduced in the dry season, a 

 point on which Dr. Gadow gives us no information. 



The elements of coloration in the skin of Amphibia, especi- 

 ally of Anura are similar to those of the skin of fishes : 

 they are of three kinds, black chromatophores, coloured 

 chromatophores, and bodies composed of reflecting substance 

 called iridocytes. Diffuse pigment also occurs and is usually 

 yellow ; the coloured pigment ranges in tint from yellow to 

 red ; as in fishes there is no green pigment, although tree- 

 frogs, like many fishes, exhibit a distinct green colour. Am- 

 phibia, like fishes, have the power of changing their colour in 

 accordance with their surroundings, a power which has been 

 lost almost entirely by reptiles, birds and mammals ; it is, how- 

 ever, retained by some reptiles and is highly developed in the 

 chamaeleons. The change of colour with change of surroundings 

 is conspicuously seen in tree-frogs ; when they sit on green leaves 

 they are green, when they sit on brown bark they become 

 brown. (Plate XIX., A.) Dr. Gadow describes how he re- 

 ceived a number of Hyla arborea in a box with moss ; 

 when they were unpacked they were of a dull greenish-grey, 

 they were placed in an enclosure with freshly cut branches 

 of a lime tree, and next morning the leaves had withered 

 and the frogs were at first invisible, but a closer examina- 

 tion showed that they were sitting on the dark brown 

 branches and had turned to a light brown colour mottled 

 with darker patches ; this is an instance of variable pro- 

 tective resemblance. According to the researches of Bieder- 

 mann the colour elements in this frog are a mosaic of 

 polygonal iridocytes or interference cells and black branched 

 chromatophores. The interference cells are stated to consist 

 of a lower half containing white granules and an upper half 

 containing yellow drops, although it has usually been found in 

 fishes that the coloured chromatophores are distinct from the 

 iridocytes. When the black chromatophores are expanded, 

 the frog is of some shade of brown, when they are contracted 

 the light passes through the yellow colour and then, being re- 

 flected from the white reflecting substance, by interference 

 becomes green. According to Biedermann tree-frogs turn 

 green when in contact with leaves even in a perfectly dark 



