652 PHYLUM CHORDATA I CLASS AMPHIBIA 



mouth shut, nose shut, eyes shut — and breathe through 

 their skin. 



Form and external features. — The absence of neck and 

 tail, the short fore-limbs almost without thumbs, the longer 

 hind-limbs with five webbed nailless toes and with a long 

 ankle region, the apparent hump-back where the hip-girdle 

 is linked to the vertebral column. There is a very rudi- 

 mentary thumb, and there is a horny knob at the base of 

 the hallux or " great toe." At pairing time the skin of the 

 first finger is modified in the males into a rough cushion, 

 darkly coloured in R. temporarta. 



The wide mouth, the valvular nostrils, the protruding 

 eyes, the upper eyelid thick, pigmented, and slightly 

 movable, the lower rudimentary and immovable, the third 

 eyelid or nictitating membrane semi-transparent and 

 moving very freely, the circular drum of the ear, the 

 slightly dorsal cloacal aperture. 



Skin. — The smooth, moist skin is loosely attached at 

 intervals to the muscles by bands of connective tissue, 

 which form the boundaries of over a score of lymph-sacs. 

 These contain fluid partly absorbed through the skin, and 

 open into the veins by two pairs of lymph-hearts. The skin 

 consists of a two-layered (ectodermic) epidermis, and an 

 internal (mesodermic) dermis. The transparent outer 

 layer of the epidermis is shed periodically, and swallowed 

 by the frog. The dermis differs markedly from that of a 

 fish, for there is no exoskeleton, though this was present 

 in the extinct Labyrinthodonts ; there are multicellular 

 glands, whose secretion keeps the skin moist and is in part 

 poisonous ; and there is a stratum of unstriped muscle 

 fibres. Pigment cells occur in the dermis, and some 

 extend between the cells of the epidermis. The frog's 

 colour changes considerably according to the distribution 

 of the pigment granules in these cells. In dark sur- 

 roundings the pigment cells form a dense network through- 

 out the skin, giving it a dull blackish hue. With increasing 

 light the granules of pigment run together into smaller and 

 more compact masses, and the skin shows lighter and more 

 yellow, especially in sunlight. Temperature and moisture 

 play an essential part in a frog's change of colour, but the 

 alteration in the distribution of pigment granules is con- 



