xin 



PHYLUM CHORDATA 



257 



1. EXAMPLE OF THE CLASS. THE COMMON FROG (Rana 

 temporaria), OR THE EDIBLE FROG (Rana esculenta). 



Rana temporaria is the common British species of Frog, found in 

 ponds and damp situations all over the country, and occurring also 

 in America ; R. esculenta is the large green edible Frog found on the 

 continent of Europe and occasionally in England ; R. pipiens is the 

 commonest North American species of the genus. Other species 

 of the same genus occur in all parts of the world except New 

 Zealand, the southern part of South America, and the various 

 oceanic islands. 



External Characters. 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 

 extra ordinary 

 width, the gape ex- 

 tending consider- 

 ably behind the 

 eye. On the dorsal 

 surface of the snout 

 are the small nos- 

 trils ; the eyes are 

 large and promi- 

 nent, and each is 

 provided with an 

 upper eyelid in the 

 form of a thick fold 

 of skin, and a nicti- 

 tating 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. The limbs are of 

 very unequal size. The fore-limbs are short, and each consists of 

 an upper arm, which, in the ordinary position, is directed back- 

 wards and downwards from the shoulder-joint ; a, fore-arm, directed 

 downwards and forwards from the elbow ; and a hand, ending in 

 four short, tapering digits, directed forwards. The hind-limb is of 



FIG. 932. Rana temporaria. (From Mivart.) 



