AMPHIBIA. 137 



As the tadpole attains its full development, the suctorial 

 organs behind the mouth gradually atrophy. The alimentary 

 canal, which is (fig. 81) at first short, rapidly elongates, and fills 

 up with its numerous coils the large body cavity. In the mean- 

 time, the lungs develop as outgrowths from the oesophagus. 



Various features in the anatomy of the Tadpole point to its being a 

 repetition of a primitive vertebrate type. The nearest living representative 

 of this type appears to be the Lamprey. 



The resemblance between the mouths of the Tadpole and Lamprey is 

 very striking, and many of the peculiarities of the larval skull of the Anura, 

 especially the position of the Meckelian cartilages and the subocular arch, 

 perhaps find their parallel in the skull of the Lamprey 1 . The internal 

 hypoblastic gill-sacks of the Frog, with their branchial processes, are 

 probably equivalent to the gill-sacks of the Lamprey 2 ; and it is not 

 impossible that the common posterior openings of the gill-pouches in Myxine 

 are equivalent to the originally paired openings of the branchial sack of the 

 Tadpole. 



The resemblances between the Lamprey and the Tadpole appear to me 

 to be sufficiently striking not to be merely the results of more or less similar 

 habits ; but at the same time there are no grounds for supposing that the 

 Lamprey itself is closely related to an ancestral form of the Amphibia. In 

 dealing with the Ganoids and other types arguments have been adduced to 

 shew that there was a primitive vertebrate stock provided with a perioral 

 suctorial disc ; and of this stock the Cyclostomata are the degraded, but at the 

 same time the nearest living representatives. The resemblances between the 

 Tadpole and the Lamprey are probably due to both of them being descended 

 from this stock. The Ganoids, as we have seen, also shew traces of a 

 similar descent ; and the resemblance between the larva of Dactylethra 

 (fig. 83), the Old Red Sandstone Ganoids 3 and Chimasra, probably indicates 

 that an extension of our knowledge will bring to light further affinities 

 between the primitive Ganoid and Holocephalous stocks and the Amphibia. 



Metamorphosis. The change undergone by the Tadpole in 

 its passage into the Frog is so considerable as to deserve the 

 name of a metamorphosis. This metamorphosis essentially 

 consists in the reduction and atrophy of a series of provisional 

 embryonic organs, and the appearance of adult organs in their 



1 Vide Huxley, " Craniofacial apparatus of Petromyzon." Journ. of Anat. and 

 Phys. Vol. X. 1876. Huxley's views about the Meckelian arch, etc., are plausible, 

 but it seems probable from Scott's observations that true branchial bars are not 

 developed in the Lamprey. How far this fact necessarily disproves Huxley's views is 

 still doubtful. 



" Conf. Huxley and Gotte. a Cf. Parker (No. 107). 



