THE FROG 581 



nate buried in the mud of the ditches and ponds, 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 jR. temporaries. 



The wide mouth, the valvular nostrils, the protruding 

 eyes, the upper eyelid thick, pigmented, and slightly mov- 

 able, 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 colour 

 changes a little according to the state of these cells, the 

 protoplasm expanding and contracting partly through the 

 direct influence of light and moisture on the skin, partly by 

 a more complex reflex action in which the eyes, the brain, 

 and the sympathetic nervous system are all implicated. In 

 the larval salamander the pigment cell seems to contract 

 and expand as a whole, but this is not usually the case. 

 There are cutaneous blood vessels, by means of which the 

 frog can, to a certain extent, breathe by its skin. The 



