88 



INTRODUCTION. 



Fig. 34. 



-sp 



unbranclied or bifid in Bufo viridis and calamita and in 

 Hyla arborea. 



In Alytes, within the egg, the external gills are 

 extremely developed, but single; and the very long 

 and slender branches are confined to the ventral side, 

 as in other tailless Batrachians. 



On entering the second period, 

 or true tadpole stage, the external 

 characters of which will be more 

 fully described in the following 

 chapter, an opercular fold covers 

 the external gills (the right some 

 time before the left in the forms 

 with lateral spiraculum), which 

 atrophy and are replaced by in- 

 ternal ones, small branched fila- 

 ments disposed along four cartila- 

 ginous arches. The anal tube is 

 developed ; the mouth acquires 

 horny mandibles and the lips 

 horny teeth ; the nostrils assume 

 a more dorsal position ; the sub- 

 buccal adhesive organ disappears ; 

 and the opercular fold having fused 

 with the skin above the gill-arches, 

 the head becomes confluent with 

 the globular swollen belly, in which 

 the extremely elongate intestine 

 shows through the transparent 

 abdominal membrane, coiled up 

 like a watch-spring (Fig. 34). 

 In the third period the hind limbs appear as buds 

 at the base of the tail, and gradually attain their full 

 development during the tadpole life. The fore limbs 

 grow simultaneously, and even more rapidly, but 

 remain concealed within a diverticulum of the bran- 

 chial chambers until fully formed, when they burst 

 through the skin (unless the left spiraculum be utilised 

 for the egress of the corresponding limb), leaving in 



Young tadpole of Hyla 

 arborea, seen from 

 below, showing the 

 branchial arches {ba), 

 the heart (c), and the 

 coiled-up intestine 

 (i). X 3. 



