408 MANUAL OF ZOOLOGY SECT. 



tadactyle type, or type of the cheiropterygtum, the Amphibia 

 differ from the fishes in the entire absence of fin-rays, and 

 from all fishes but the Dipnoi, in the presence of lungs for 

 breathing air in the adult : the larval gills become absorbed 

 in the majority of Amphibia before maturity is reached, but 

 in some are retained throughout life, the animal breathing, 

 like the Dipnoi, by means of both lungs and of gills. 



The most convenient example of the Amphibia for special 

 study is a frog. The following description and figures have 

 reference more specially to the European species of the 

 genus Rana, R. temporaries and R. esculenta, but they 

 will be found to apply to any American frog or toad, except 

 in a few features which are chiefly quite superficial, some of 

 which will be referred to subsequently. 



The trunk is short and stout, and is continued, without 

 the intermediation of a neck, into the broad depressed head. 

 There is no trace of a tail, the anus being terminal. The 

 mouth also is terminal, and is characterised by its ex- 

 traordinary width, the gape extending considerably behind 

 the eye. On the dorsal surface of the snout are the small 

 nostrils ; the eyes are large and prominent, and each is pro- 

 vided with an upper eyelid in the form of a thick fold of skin 

 and a nictitating membrane a much thinner fold which 

 arises from the lower margin of the eye and can be drawn 

 up over it. Close behind the eye is a circular area of 

 tensely stretched skin, the tympanic membrane, a structure 

 not met with in any fish ; as we shall see, it is an accessory 

 part of the auditory organ. There is no trace of branchial 

 apertures. 



The back has a peculiar bend or hump, in the sitting 

 posture, marking the position of the sacral vertebra (see 

 p. 410). The limbs are of very unequal size. The fore- 

 limbs are short, and each consists of an upper arm, which, 



