428 MANUAL OF ZOOLOGY SECT. 



or tadpole with two pairs of branched external gills and a 

 long tail (Fig. 242). There is no mouth, and eyes have not yet 

 become developed. On the lower surface of the head is a 

 pair of suckers by which the tadpole is able to attach itself to 

 water-weeds. Soon a third pair of external gills appears, the 

 mouth and gill slits are formed, and the eyes appear. The 

 mouth is small, bounded by lips beset with horny papillae 

 and provided with a pair of horny jaws. The enteric canal 

 grows to a great length and is coiled like a watch-spring, and 

 the tadpole browses upon the water-weeds which form its 

 staple food. 



Soon the external gills show signs of shrivelling, and at 

 the same time internal 'gills , like those of Fishes, are developed 

 on the branchial arches. A fold of skin, the operculum, 

 appears on each side, in front of the gills, growing from the 

 region of the hyoid arch : this extends backwards until the 

 gill-slits and external gills are covered and there is only a 

 single small external branchial aperture on each side. On 

 the right side the operculum soon unites with the body-wall 

 so as to close the branchial aperture, but on the left side 

 the opening remains for a considerable time as the sole 

 exit of the water. At this time the tadpole is to all intents 

 and purposes a Fish. 



The lungs now appear, and the larva is for a time truly 

 amphibious, rising periodically to the surface to breath air ; 

 the single branchial aperture, however, soon closes, and 

 henceforth respiration is purely aerial. 



In the meantime the limbs are developed. The hind 

 limbs appear as little rounded buds, one on each side of the 

 root of the tail. The fore limbs arise beneath the operculum 

 and are therefore hidden at first ; soon, however, they 

 emerge by forcing their way through the operculum. As 

 the limbs increase in size the tail undergoes a progressive 



