116 COMPARATIVE ZOOLOGY. 
breathing by the skin and breathing by lungs are one in 
principle. Indeed, in many animals, especially Frogs, res- 
piration is carried on by both lungs and skin. 
All Vertebrates have two kinds of respiratory organs in 
the course of their life. From Fish to Man, all have gills 
in the embryo state.” Jishes, and a few 
Amphibians, keep them through life; but 
in the rest they disappear. All, too, have 
lungs; but fully developed only in Reptiles, 
Birds, and Mammals. The lung of the Fish 
(the air-bladder) remains rudimentary. A 
few adult Amphibians, as Proteus and Si- 
ren, retain both gills and lungs, thus forming 
a link between Fishes and Reptiles. But 
Frogs and Salamanders begin life as water- 
breathers, and when mature have lungs only. 
The lungs of Vertebrates are elastic mem- 
branous sacs, divided more or less into cells 
to increase the surface. Upon the walls of 
the cells are spread the capillary blood-ves- 
sels. The smaller the cells, the greater the 
extent of surface upon which the blood is 
exposed to the influence of the air, and, 
therefore, the more active the respiration 
and the purer the blood. The lungs are 
relatively largest in Reptiles, and smallest 
ear he Gytte in Mammals. But in the cold-blooded Am- 
Peapentleh ts phibians and Reptiles, the air-cells are few 
bifurcation; ¢, and large; in the warm-blooded Birds and 
aed pu. Mammals, they are exceedingly numerous 
tile “aie: aa and minute.” In Birds and Mammals, the 
rudimentary. blood in the capillaries is exposed to the air 
_on all sides; in the Reptiles, on one only. Respiration is 
most perfect in Birds; they require, relatively to their 
weights, more air than Mammals or Reptiles, and most 
