EXTERNAL FEATURES OF FROG. 531 



Though aquatic in youth, frogs often live in dry places, 

 hiding in great drought, reappearing when the rain returns. 

 Everyone knows how they sit with humped back, how they 

 leap, how they swim. They feed on insects and slugs. 

 These are caught by the large viscid tongue, which, being 

 fixed in front of the mouth and free behind, can be jerked 

 out to some distance, and with even greater rapidity 

 retracted. When we watch a frog, we see that the nostrils 

 are alternately opened and closed, and that the under side 

 of the throat is rhythmically expanded and compressed, the 

 mouth remaining shut meanwhile; the movements are 

 evidently connected with respiration. That the males 

 trumpet in the early spring to their feebly responsive mates, 

 that in our British species the pairing takes place soon after, 

 that the young are tadpoles, that a notable metamorphosis 

 takes place, are familiar facts of observation. In winter the 

 frogs hibernate, buried in the mud of the pond, and breath- 

 ing through their skin. 



Form and External Features. 



We notice 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 rudimentary 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. temporaria. 



We see the wide mouth, the paired nostrils, the projecting 

 eyes, the upper eyelid thick, pigmented, and almost im- 

 movable, the lower semi-transparent and moving very 

 freely, the circular drum of the ear, the smooth skin, with 

 patches of a deeper tint on its yellowish ground, and the 

 slightly dorsal cloacal aperture. 



Skin. 



The smooth, moist skin is but loosely attached to some 

 parts of the body ; it consists of an external two-layered 

 (ectodermic) epidermis, and an internal (mesodermic) dermis. 

 The outer layer of the epidermis is shed periodically. The 



