gills. In the next order, that of the Tritons and Salamanders, the tails are retained, but the external gills are lost; finally, the Frog has neither gills nor tail, but the tadpole or the immature frog has both, so that in one stage of its existence (Fig. 62) the Frog is a Siredon, later it is more like a Salamander, finally it (Fig. 63) resembles neither.
REPTILES.
Leaving the Fishes and Batrachia, and turning to the Reptiles, we see that the Fishes and Batrachia breathe by means of gills (the Batrachia at some stage of their existence), whereas the Reptiles always breathe by lungs, as a bird or four-footed creature. The Vertebrates have been divided by some naturalists, for this reason, into the two divisions of the gill-breathing. Fish, Batrachia; and the lung-breathing, Reptiles, Birds, Mammals. The reptiles, birds, and mammals agree with each other in possessing, during embryo life, an amnion and an allantois (see Embryology); the fishes and batrachia never, at any stage of their existence, possess either. The amnion is a transparent sac filled with a fluid (liquor amnii) in which the young bird or reptile floats. The allantois is a vesicle starting from the under part of the body of the bird or reptile, and filling up the interior of the egg. The allantois is filled with blood-vessels; and as the porosity of the egg-shell permits the passing out of the pernicious carbonic acid, and the passing in of the life-sustaining oxygen, it is by means of the allantois that respiration is effected. The visceral arches in the young bird and reptile (Figs. 179, 178 c) are converted through growth into part of the jaws and part of the organ of hearing; the visceral arches in the fishes are modified into gills. We see, therefore, that a great progress has been made on comparing the structure of the gill- and lung-breathing division of Vertebrates.
