532 AMPHIBIA. 



The Frog as a type of Amphibians. 



The common British frog (Rana temporarid) and the 

 frequently imported continental species (R. esculenta) agree 

 in essential features. . 



Though aquatic in youth, they 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 living 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 re- 

 tracted. When a frog is breathing the nostrils are alternately 

 opened and closed, the under side of the throat is 

 rhythmically expanded and compressed, the mouth re- 

 mains shut meanwhile. The males trumpet in the 

 early spring to their feebly responsive mates. In our 

 British species the pairing takes place soon after; the 

 young are familiarly known as tadpoles, and a notable 

 metamorphosis takes place. In winter the frogs hiber- 

 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 R. temporaria. 



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 

 sacs contain fluid absorbed through the skin, and open into 



