The beak is formed of a pair of horny plates moulded on the upper and lower pairs of labial cartilages. The upper valve of the beak is the larger of the two, and covers the lower. The beak is surrounded by a projecting lip formed of a circular fold of skin, the free edge of which is covered by papillæ. Between the papillæ and the beak rows of horny teeth are placed on the inner surface of the lip. There are usually two rows of these on the upper side, the inner one not continuous across the middle line, and three or four rows on the lower side, the inner one or two divided into two lateral parts. 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 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 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[49]. The internal hypoblastic gill-sacks of the Frog, with their branchial processes, are probably equivalent to the gill-sacks of the Lamprey[50]; 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. [49] _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. [50] Conf. Huxley and Götte.