XIII 



BACKBONED ANIMALS 



277 



normally lungs and use them. The skin is characteris- 

 tically soft, naked, and clammy. Unpaired fins are 

 sometimes present on the back and tail as in Fishes, but 

 are never supported by fin-rays. 



The class includes four orders, of which the Labyrin- 



FIG. 89. THE COMMON FROG (Rana temporaria). 



The student should verify on a specimen the external features shown 

 in the drawing. When breathing, the frog keeps its mouth tightly shut, 

 and air enters by the paired valved nostrils. The nostrils of fishes are 

 exclusively olfactory ; from amphibians onwards they are olfactory and 

 respiratory. The prominent eyes have a distinct upper eyelid, and a 

 very rudimentary lower eyelid, continued, however, into a transparent 

 nictitating membrane, which can be drawn upwards over the eye. The 

 tympanum or drum of the ear is flush with the skin, not sunk inwards 

 as usuil. At a certain stage of development in higher vertebrates the 

 drum is on the surface. A hump on the back shows where the hip- 

 girdle is attached to the last free vertebra, the ninth. The frog has an 

 unusually small number of vertebrae and no ribs. There is no thumb. 

 There are no nails or claws. The hind-foot is very long and thus adapted 

 for swimming and leaping. The skin is loose, naked, glandular, pig- 

 mented, with absorptive and respiratory functions. All through the 

 winter the frog, buried in the mud, breathes cutaneously. 



thodonts are wholly extinct, the other three being repre- 

 sented by tail-less frogs and toads (Anura), by newts 

 and salamanders (Urodela) with distinct tails, and by a 

 few of worm-like form and burrowing habit, e.g. Ccecilia. 



