ANIMAL RELATIONSHIPS. 247 
stages of growth of higher Chordata, and never ad- 
vances beyond this embryonic condition. These illus- 
trations show us that embryonic conditions must be 
known in order to classify by relationship. 
Fic. 200.—Diagram of a Sea-squirt. «, mouth; 4, vent; ¢, gullet-open- 
ing; d, nerve-ganglion; ¢, stomach; /, test or outer layer; g, tunic 
or inner layer; 4, branchial sac. 
In studying the frog, attention was called to the fact 
that the lungs are formed by simply pushing out the 
walls of the throat. The air-bladder of a fish, and our 
own lungs, are formed in the same way. The air- 
Fic. 201.—Amphioxus. a@, mouth; 4, ¢, heart; d, liver; g, respiratory 
organs; 4-9, digestive canal; 4, notochord; m, spinal marrow; a, 
tail-fin, 
bladder of the fish is less complicated than the lung of 
the frog, and our own lung is more complicated. The 
process of growth, however, is equally simple in all 
