1 86 AMPHIBIA 



mented ring under the transparent epidermis ; the mouth be- 

 comes bordered by fleshy lips ; the anus is perforated ; and the 

 larva (we shall not yet call it a tadpole) is able to feed, having 

 thus far subsisted on the yolk contained in the abdomen. (Fig. 

 13, B, i, 2.) 



On entering the second period, or true tadpole stage, an oper- 

 cular fold covers the external gills, which atrophy and are re- 

 placed by internal ones — small branched filaments disposed 

 along cartilaginous arches. An anal tube is developed; the 

 mouth acquires horny, beak-like mandibles, and the funnel- 

 shaped lips horny teeth ; the nostrils assume a more dorsal 

 position ; the " holder " disappears ; and the opercular fold 

 having fused with the skin above the gill-arches, leaving an 

 opening (spiraculum) for the egress of the water from the gill- 

 chamber, the head becomes confluent with the globular, swollen 

 body, in which the extremely elongate gut shows through the 

 transparent abdominal membrane, coiled up like a watch-spring. 

 (Fig. 13, B, 4, 5, 6.) 



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. (Fig. 13, B, 5.) The fore limbs grow 

 simultaneously, and even more rapidly, but remain concealed 

 within a diverticulum of the branchial chambers until fully 

 formed, when they burst through the skin (unless the left 

 spiraculum be utilised for the egress of the corresponding 

 limb). (Fig. 13, B, 6.) Then only the caudal fin-membrane 

 becomes reduced and the tail is gradually absorbed ; the 

 gills entirely disappear; the lungs, which had co-existed 

 as accessory respiratory and hydrostatic organs, assume alone 

 (or, rather, together with the skin) the respiratory function ; 

 the horny armature of the mouth and lips is shed in pieces; 

 the lips are absorbed and the buccal cleft extends ; the eyes 

 become free and acquire movable lids; the intestine shortens; 

 and the young frog, usually still bearing a stumpy tail, leaves 

 the water. (Fig. 13, B, 7.) The metamorphosis is completed. 



Such is, briefly sketched, the development of the typical 

 frog or toad. But there are many exceptions to this course, 

 especially among species living between the tropics ; and we 

 shall now deal with some of the more interesting examples with 

 which we are at present acquainted. 



