THE STUDY OF AMPHIBIANS 125 



EXTERNAL FEATURES 



As an exercise in precision of observation a study may be made 

 of the frog's external features. In some cases it may be possible 

 to suggest an interpretation of the peculiarities. 



The absence of any neck region and of a tail ; the short fore- 

 limbs, almost thumbless ; the swollen first finger, which is one 

 of the marks of a male frog ; the strongly developed hind-limbs 

 with webbed feet and very long ankle region, in obvious adapta- 

 tion to swimming and jumping ; the absence of nails on fingers 

 or toes ; the horny knob at the base of the big toe ; the apparent 

 hump-back where the hip girdle is linked to the backbone. 



The wide mouth, the valved nostrils, the protruding eyes, 

 the upper eyelid thick, pigmented, and slightly movable, the 

 lower eyelid rudimentary and not movable, a third transparent 

 eyelid drawn up from beneath and moving very freely, the cir- 

 cular drum of the ear flush with the skin, the slightly dorsal 

 cloacal aperture. 



It may be interesting to take up in detail some structure 

 such as the skin, which may be contrasted with the correspond- 

 ing structure in fishes and in higher animals. The frog's skin 

 is very loose, there are large lymph spaces between it and the 

 subjacent muscles. The transparent outermost layer of the 

 epidermis dies away periodically and is shed, the pieces being 

 swallowed by the frog. There is no hint of scales. There are 

 numerous glands, whose secretion keeps the skin moist and makes 

 it unpalatable. The skin contains abundant pigment cells, and 

 the colour can change a little according to temperature, illumina- 

 tion, etc. The frog can breathe through its skin and absorb 

 water through its skin. A higher animal can do neither. 



THE FROG A " COLD-BLOODED " ANIMAL 



The frog may be taken as a convenient starting-point for 

 some inquiry into an interesting but difficult matter, the 

 difference between " warm-blooded " and " cold-blooded " 



