ZOOLOGY. 61 
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 exist- 
ence (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 exist- 
ence), 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 hear- 
ing; 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. 
