HOW ANIMALS BREATHE. 
117 
Like the alimentary canal, the lungs of an animal are 
really an inflected portion of the outer surface; so that 
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. 
The lungs of Vertebrates are derived from the front 
part of the alimentary canal. In some Fishes, air is swal¬ 
lowed, which passes the whole length of the digestive 
tract, and is expelled from the anus. Here 
the whole canal serves for respiration. In 
Reptiles, Birds, and Mammals the hinder 
part of the intestine develops an outgrowth 
(the allantois) during embryo-life which 
serves as the embryo’s breathing organ (Figs. 
170, 171). 
All Vertebrates have two kinds of respir¬ 
atory organs in the course of their life. 
Fishes have gills; their lung (the air-blad¬ 
der) rarely serves as a functional respiratory 
organ, and is sometimes wanting. Amphibi¬ 
ans have gills while in the larval state. Some 
keep them throughout life; but all develop 
functional lungs, and also breathe by means 
of the skin. 
In the remaining Vertebrates, the allantois 
is the breathing organ of the embryo, and 
the lung is the breathing organ of the adult. 
The skin is of small or no importance in 
respiration. 
The lungs of Vertebrates are elastic mem¬ 
branous sacs, divided more or less into cavities of a suake: a, 
(the air-cells) to increase the surface. Upon bifurcation; c, 
the walls of the air-cells are spread the capil- 
lary blood-vessels. The smaller the cells, the »«y «■» ^ 
greater the extent of surface upon which the mentary. 
