AMPHIBIA. 



Ill 



two or thi'ee peritoneal openings, opposite to which a glomeruhis is 

 formed. 



The mesonephros (permanent kidney of Amphibia) is formed as a series 

 of segmental tubes much later than the pronephros, during late larval life. 

 Its anterior end is situated some distance behind the pronephros, and during 

 its formation the pronephros atrophies. 



The period of hatching varies in different larvae, but in most 

 cases, at the time of its occurrence, the moiith has not yet become 

 perforated. The larva, familiarly known as a tadpole, is at first 

 enclosed in the detritus of the gelatinous egg envelopes. The tail, 

 by the development of a dorsal and ventral fin, very soon becomes a 

 powerful swimming organ. Growth, during the period before the 

 larva begins to feed, is no doubt carried on at the expense of the 

 yolk, which is at this time enclosed within the mesenteron. 



The mouth and anal perforations are not long in making their 

 appearance, and the tadpole is then able to feed. The gill slits also 

 become perforated, but the hyomandibular diverticulum in most 

 species never actually opens to the exterior, and in all cases becomes 

 very soon closed. 



There can be but little doubt that the hyomandibular diverticulum gives 

 rise, as in the Amniota, to the Eustachian tube and tympanic cavity, 

 except when these are ab- 

 sent (i.e. Bombinatoridfe). 

 Gotte holds however that 

 these parts ai-e derived 

 from the hyobranchial 

 clefb, but his statements 

 on this head, which would 

 involve us in great morplio- 

 logical difficulties, stand in 

 direct contradiction to the 

 careful researches of Par- 

 ker. 



Shortly after hatch- 

 ino- there grows out from 

 the hyoid arch on each 

 side an opercular fold of 

 skin, which gi'adually 

 covers over the posterior 

 branchial arches and the 

 external gills (fig. 80 d). 

 It fuses with the skin 

 at the upper part of 

 the gill arches, and also 

 with that of the peri- 

 cardial wall below them ; 

 but is free in the mid- 

 dle, and so assists in 



Fig. 80. Tadpoles with external branchi^. 

 (From Huxley; after Ecker.) 



Lateral view of a young tadpole. 

 Ventral view of a somewhat older tadpole, 

 external branchife ; m. mouth ; n. nasal sack ; 

 o. auditory vesicle; z. horny jaws; s, ventral 

 sucker; d. opercular fold. 



C. More advanced larva, in which the opercular 

 fold has nearly covered the branchiffi. 



s. ventral sucker; kn. external branchiae; y. rudi- 

 ment of hind limb. 



A. 

 B. 



Teh. 

 a. eye 



