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. 
Fig. 200.— Diagram of a Sea-squirt, a, mouth; b, vent; c , gullet-open¬ 
ing; d, nerve-ganglion; e, stomach; /, test or outer layer; g , tunic 
or inner layer; h, 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- 
Fig. 201. —Amphioxus. a , mouth; b , c, heart; d, liver; g, respiratory 
organs; h-p , digestive canal; /, notochord; m, spinal marrow; o , 
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 
