INTRODUCTION. 5 



Sub-class I. Elasmobranchii. 



Fishes with cartilaginous skeleton, mouth usually on the lower 

 side of the head, the gills usually opening separately on the neck, and the 

 tail with the upper lobe the larger (heterocercal). Sharks and skates. 

 The Holocephali differ in having the gill slits covered with a fold of 

 skin, so that but a single external opening appears. 



Sub-class II. Ganoidea. 



Intermediate between elasmobranchs and teleosts. Garpike, 

 sturgeon. 



Sub-class III. Teleostei. 



Fishes with bony skeleton, mouth with true jaws at the tip of the 

 snout, gill openings concealed by an operculum or gill-cover supported 

 by bone. Tail with upper and lower lobes equal. All common fishes. 



Sub-class IV. Dipnoi. 



The lung fishes are tropical forms in which the air bladder func- 

 tions as a lung, the gill openings are covered with an operculum, and 

 the tail is very primitive (diphy cereal). 



Class II. Amphibia. 



Ichthyopsida with legs replacing the paired fins, lungs 'present and 

 replacing the gills in the adult, nostrils connecting with the mouth. 



Sub-class I. Stegocephali. 

 Extinct amphibians with well developed tail. 



Sub-class II. Urodela. 



Amphibia with well developed tail, gills sometimes retained through 

 life. Salamanders, Tritons, newts, efts. 



Sub-class III. Anura. 



Tailless as adults, the young a tadpole with external gills. Frogs 

 and toads. 



Sub-class IV. Gymnophiona. 



Blind, burrowing, legless amphibians occurring in the tropics. 

 Caecilians. 



