EXTERNAL CHARACTERS OF THE FROG. 447 



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 carefully, we notice 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 concerned with respiration. That 

 the males trumpet in the early spring to their weakly re- 

 sponsive mates, that in our British species the pairing takes 

 place soon after, that the young are tadpoles, that a notable 

 metamorphosis takes place, are famiUar facts of observa- 

 tion. In winter the frogs hibernate, buried in the mud of 

 the pond. 



Form and External Features. — We notice the absence of 

 neck and tail, the shorter fore-limbs almost without thumbs, 

 the longer hind-limbs with five webbed nail-less 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 innermost digit of the fore-limb becomes modified in 

 the males into a rough cushion, which is darkly coloured in 

 R. temporaria. 



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

 eyes with well-developed lids, the circular drum of the ear, 

 the smooth yellowish skin with patches of other colours, 

 and the cloacal aperture. 



Skin. — The smooth, moist skin covers some parts of the 

 body loosely ; it consists of an external two-layered (ecto- 

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

 The outer layer of the epidermis is shed periodically. The 

 dermis differs markedly from that of a fish, for there is no 

 exoskeleton ; there are multicellular glands, whose secretion 

 makes the skin moist ; and there is also a stratum of un- 

 striped muscle-fibres. In the dermis there are also branched 

 pigment-cells, usually in two strata. Through a reflex nervous 

 action they are slightly affected by the colour of the surround- 

 ings, Furthermore, there are cutaneous blood-vessels, by 

 means of which the frog can to a certain extent breathe 



