THE FROG. 561 



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. tonporaria. 



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 colour changes a little accord- 

 ing 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 tadpole has sensory cells in distinct 



36 



