INTRODUCTION 5 



Grade I. Ichthyopsida (Anamnia) 



Fish-like, breathe, at least while young, by gills, have paired ap- 

 pendages, in the shape of legs or fins. In development there are 

 never formed those structures to be described later as amnion and 

 allantois. 



Class I. Pisces 



Fishes respire permanently by gills developed in gill slits in the 

 sides of the pharynx, have median and paired fins unless the latter be 

 lost by degeneration. 



Sub-class I. Elasmohranchii 



Fishes with cartilaginous skeleton, mouth usually on the lower 

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

 (skates) on the lower surface, 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, nostrils open into mouth, the gill openings are covered 

 with an operculum, and the tail is very primitive (diphycercal). 



Class II. Amphibia 



Ichthyopsida with legs replacing the paired fins, usually external 

 gills in the young, lungs present and replacing the gills in the adult, 

 nostrils connecting with the mouth. 



