AMPHIBIA. 113 



numerous coils the large body cavity. In the meantime, 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 representa- 

 tive 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 ai-ch, per- 

 haps find their parallel in the skull of the Lamprey'. The internal hypo- 

 blastic gill-sacks of the Frog, with their bi-anchial processes, are probably 

 equivalent to the gill-sacks of the Lamprey* ; and it is not impossible that 

 the common posterior openings of the gill-pouches in Myxine are equiva- 

 lent 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 sti-iking not to be mei'ely 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 pro- 

 vided 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^ and 

 Chimsera, 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 re- 

 duction and atrophy of a series of provisional embryonic organs, and 

 the appearance of adult organs in their place. The stages of this 

 metamoi-phosis are shewn in fig. 82, 5, 6, 7, 8. 



The two pairs of limbs appear nearly simultaneously as small 

 buds^; the hinder pair at the junction of the tail and body (fig. 82, 5), 

 and the anterior pair concealed under the opercular membrane. The 

 luugs acquire a greater and greater importance, and both branchial 

 and pulmonary respirations go on together for some time. 



When the adult organs are sufiiciently developed an ecdysis takes 

 place, in which the gills are completely lost, the provisional horny 

 beak is thrown off, and the mouth loses its suctorial form. The 

 eyes, hitherto concealed under the skin, become exposed on the 



^ Vide Huxley, "Craniofacial apparatus of Petromyzon." Journal 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 stiU doubtful. 



'^ Conf. Huxley and Gotte. ^ Cf. Parker (No. 107). 



B. E. II. 8 



