Chap. XIV.] 



TADPOLES. 



clefts, we have a creature with a long tail, a sucking 

 mouth with horny jaws, and two suckers below, and 

 with external gills. In a later stage of metamorphosis 

 the gills become covered by an operculum (o) \ two 

 buds between the body and the tail are the rudiments 

 of the future hind limbs ; later on the signs of fore 



n 



n 



e 



Fig. 217. Structure of the Tadpole. 



A, From the side ; B, from below ; c, a later stage ; n, Lead of tadpole ; g pi gi, 

 gills ; m, mouth ;j,ja\vs ;n, nasal sac ;e, eye ;a, car ; o, operculum ; hi, rudiment 

 of hind limbs ; s, sucker ; Ip, upper lip ; cJi to cl*>, visceral clefts. 



limbs become apparent, the gills are lost, and their 

 clefts disappear, while the tail gradually undergoes 

 atrophy, and the larva is converted into a small 

 quadruped tail-less frog, which breathes air by means 

 of lungs. A similarly tailed larval stage obtains in 

 some of the Urochordata, which, in adult life, are fixed. 

 Among the Arthropoda, Insects, as we well 

 know, present, when their metamorphosis is "com- 

 plete," three distinct stages ; in the earliest stage, or 

 that of the " larva" (Fig. 218 ; A), the product of the 



