VARIATION AND ADAPTATION 211 



are entirely wanting. This structure characterises what may be 

 called the first stage of larval life. After about a week in the 

 frog and its allies the second stage begins in which the external 

 gills disappear and a transverse flap of membrane grows back- 

 wards from the hyoid arch and covers up the gill-clefts. This 

 membrane is known as the " operculum," or " opercular fold ". 

 In the frog the posterior edges of the opercular folds unite 

 with the skin of the body behind the gill-clefts leaving only a 

 small opening on the left side. This aperture is sometimes 

 called the spiracle, but it does not of course correspond to the 

 spiracle of the shark-tribe which is represented in the frog by 

 the tympanic chamber or ear-cavity ; the latter communicates 

 with the throat by the Eustachian aperture but is closed ex- 

 ternally by the tympanic membrane. In the tadpoles of the 

 Aglossa, Pipa and Dactyletkra, there are two " opercular aper- 

 tures " one on each side, and in the Discoglossidae the two lateral 

 apertures unite into one which is ventral and median. In the 

 tadpoles of the Urodelathe operculum is always small and rudi- 

 mentary, it extends only a short distance from the hyoid arch, 

 and does not cover the external gills ; the latter are more dorsal 

 in position than in the Anura and the opercular fold is sometimes 

 continuous with the base of the first gill. The two opercular 

 folds meet below the throat and often persist in the adult as a 

 transverse ridge of skin known as the gular fold. Considering 

 the structure of the gills, or branchial organs, in Dipnoi (lung- 

 fish) and Crossopterygian (fringe-finned) fishes from which the 

 Amphibia must have been originally descended, the rudimen- 

 tary operculum in Urodela cannot be regarded as a primitive 

 feature, but we must conclude that in the course of evolution 

 the ancestral operculum has undergone divergent modifications 

 in the Urodela and the Anura, in the former having been 

 reduced, in the latter forming a closed branchial chamber with 

 the exception of the small apertures above described. 



Considerable difference exists between the Anura (frog- 

 tribe) and Urodela (newts and salamanders) in the metamor- 

 phosis with respect to the limbs and tail. In the Anura the 

 fore-limbs are at first concealed beneath the operculum, that 

 on the left protruding through the branchial aperture, that of 

 the right side actually bursting through the operculum. The 

 hind limbs grow much faster than the fore-limbs ; although 



