298 



CLASS AMPHIBIA. 



forms (p. 289) as the lungs develop. In the so-called perenni- 

 branchiate forms the gills and lateral line persist, either partly 

 or throughout life, but the pronephros always gives place to 

 the kidney. In some cases the larvae actually become sexually 

 mature (paedogenesis, p. 280). In the Anura the larval organs 

 are more conspicuous and the final change by which the aquatic 

 tadpole becomes a terrestrial animal is so striking as to be called a 

 metamorphosis. The tadpole possesses two suckers (Fig. 170, 8) 

 on the ventral surface behind the mouth, which however disappear 



quite early in larval 

 ^^ -^ life (similar suckers 



are present on the 

 throat of Triton larvae, 

 where however they 

 are stalked). It also 

 possesses a horny beak 

 (Hz) on each lip which 

 is lost at the metamor- 

 phosis. Moreover the 

 caudal region of the 

 body is represented 

 by a well developed 

 laterally compressed 

 tail, which is gradually 

 absorbed after the 

 animal has become 

 terrestrial, and the 

 limbs functional. The 

 limbs develop during 

 larval life : the anterior limbs are formed beneath the opercular 

 fold and only appear at the metamorphosis, when that membrane 

 is shed ; the posterior limbs develop at the side of the anus and 

 are visible from their first appearance as buds. The operculum 

 develops early in larval life as a cutaneous fold from the hyoid 

 arch which gradually grows back over the gills and gill-slits, 

 causing the atrophy of the external gills (Fig. 170). A branchial 

 chamber is thus formed between the fold and the body, into 

 which the gill-slits open. These chambers at first open widely 

 behind, but very soon the openings narrow. In the Aglossa both 

 openings persist and are lateral in position. In the Disco- 



FiG. 170. — Larval stages of the frog (after Ecker). a, 

 embryo some time before hatching, with wart-like gill 

 papillae on the branchial arches. 6, larva some time 

 after hatching with external gills, c, older larva with 

 horny beak and small branchial clefts beneath the 

 integimientary operculum, with internal gills. N nasal 

 pit ; S sucker ; K external gills ; A eye ; Hz horny 

 tooth. 



