BLOOD SYSTEM. 



37 



from their bony, enameled scales, which form a corr 



plete external armor. The)' supplement their gill breath 



ing by a little air breathing. 



They were abundant in early 



geological times, but are now 



represented by the Lepidosteus, 



Polypterus, etc. (3) Teleosts 



(perfect bone), or ordinary 



bony fishes. They breathe 



wholly by gills, but in some 



the air bladder opens into the 



throat. (4) Elasmobranchs 



(plate gills), represented by 



sharks, skates, rays, etc. (5) 



Marsipobranchs (pouch gills), 



represented by the lampreys 



or conger eels. And (6) the 



pharyngobranchs (throat gills), 



represented only by that strange creature the amphioxu 



or lancelet. The schedule will express this classifica 



tion in brief space : 



1. Dipnoi — lung fishes. 



2. Ganoids — armored fishes. 



3. Teleosts — typical fishes. 



4. Elasmobranchs — sharks, skates, etc. 



5. Marsipobranchs — lampreys. 



6. Pharyngobranchs — lancelet. 



Fig. 256.— A portion of the ii 

 terior of the air bladder of 

 Lepidosteus (enlarged), shov 

 ing its cellular structure. 



TRANSITION FROM GILL BREATHING TO LUNG 

 BREATHING. 



We have seen pure gill breathing in teleosts am 

 lower fishes. We saw pure lung breathing attained ii 

 the higher amphibians, as frogs, etc. Now, in the highe 

 fishes and the lower amphibians we find every gradatioi 

 between — not, indeed, by transformation of the one int 



