376 ANIMAL PHYSIOLOGY. 
rious classes of animals, whether terrestrial or aquatic, evident 
without extended treatment of the subject in the text. What 
Fie. 309. 
Fic. 309.—A. Pulmonary sac. B. respiratory leaflets of Scorpio occitanus (after Blanchard). 
Fie. 310.—Left pulmonary sac, viewed from dorsal aspect, of a spider (after Dugés). pm, 
pulmonary lamelle ; stg, stigma, or opening to former. 
we are desirous of impressing is that throughout the entire ani- 
mal kingdom respiration is essentially the same process; that 
Fic. 311.--A. B. Tadpoles with external branchiz (after Huxley). 7, nasal sacs; a, eye; 0, 
ear; k.b, branchie ; m, mouth; z, horny jaws; s, suckers; d, opercular (or gill) fold. 
C. More advanced frog’s larva. y, rudiment of hind-limb ; k. s, single branchial aperture. 
Owing to figure not having been reversed, this aperture seems to lie:on right instead of 
left side. 
finally it resolves itself into tissue-breathing: the appropria- 
tion of oxygen and the excretion of carbon dioxide. Since the 
manner in which oxygen is introduced into the lungs and foul 
gases expelled from them in some reptiles and amphibians, is 
largely different from the method of respiration in the mam- 
mal, we call attention to this process in an animal readily 
watched—the common frog. This creature, by depressing the 
floor of the mouth, enlarges his air-space in this region and 
consequently the air freely enters through the nostrils; where- 
upon the latter are closed by a sort of valve, the glottis opened 
