Water-breathing Vertebrates 127 



ment are to be observed in newts and salamanders. In these 

 animals also there is a passing from the Qg^ to the tadpole. 

 Here also the tadpole lives in water, breathes through gills. 

 Here also the tadpole is gradually transformed into an ani- 

 mal with legs and without gills, breathing through lungs. 

 On the basis of these similarities, we group the newts and 

 salamanders with the frogs and toads as batrachians or am- 

 phibians — and assume their relationship. 



W at er -breathing Vertebrates 



The class of animals known as fishes includes the most 

 familiar and characteristic water animals. In fact, we are 

 for this reason often inclined to apply the name " fish " to 

 types of animal that are not fish at all, like jellyfish, cuttle- 

 fish and starfish — representatives of three diverse branches 

 of animal life. In common with all backboned animals, in- 

 deed with all other animals, the fish starts life as a single cell 

 — the fertilized Q^g. As in other animals this Qg^ cell 

 divides into two, and each of these again into two and so 

 on until numerous cells are formed. Hour by hour one can 

 see new cells formed, the parts gradually taking on the ap- 

 pearance of distinct structures, and after a time it is possible 

 to recognize the entire mass as resembling " fish." 



During this early period of development the successive 

 stages are remarkably like certain stages in the development 

 of the frog. It is no stretch of the imagination that makes 

 us compare the tadpole of the frog to the young fish. In 

 both animals there is the same ^general form. There is swim- 

 ming by the action of the tail. There is breathing by means 

 of gills. Internally also the tadpole resembles the fish in 

 many details. The young fish, however, remains fish-like as 

 it grows older. We may say that the adult fish is the same 

 as the young fish only more so. The tadpole, on the other 

 hand, diverges from this pattern as it grows older. The buds 

 on the side of the body, which in the case of the fish become 

 fins, grow in the tadpole into legs. The tail is gradually 



