xir PHYLUM CHORDATA 405 



dactile limbs are developed, and lung's for breathing air 

 take the place of gills as the organs of respiration,, while 

 corresponding advance to a higher type of structure oc- 

 curs in the other parts. The Amphibia thus occupy an 

 intermediate position between the Fishes on the one hand 

 and the higher air-breathing Vertebrates (Reptiles, Birds, 

 and Mammals) on the other. In addition to the possession 

 of limbs constructed on the pentadactyle type, or type of 

 the cheiropterygium, 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 both of 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. temporaria and R, escuknta 

 but they will be found to apply to any 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 

 provided 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 



