GREA T BRITAIN. 7 



oviparous or oviviparous, but in all of which gills exist 

 during some period of their existence. 



Speaking generally, fishes are vertebrate animals, adapted 

 for an aquatic life, having their extremities modified into fins. 

 TJiey breathe (except in some few and mostly tropical forms) 

 through the medium of water, solely by means of their gills. 

 They are cold blooded, tJteir heart having merely two cavities. 

 Some possess scales, others are destitute of them, or else they 

 may be modified into bony plates. 



The fishes which exist in our seas may be divided into 

 the following sub-classes, omitting the Cyclostomata (Lam- 

 preys) : 



I. TELEOSTEAN or bony forms, likewise termed "true 

 fishes," and in which the skeleton is bony and the vertebrae 

 separated one from another, the posterior ones being bony 

 or covered with osseous plates. The branchiae are free, 

 the bulb'of the aorta having simply two valves at its origin. 



II. CHONDROPTERYGIAN or cartilaginous forms, with no 

 cranial sutures, the gills attached by their outer edges to 

 the skin (see note, p. 314). 



There are two orders among the cartilaginous fishes 

 (i.) Ganoidei, having merely one external gill-opening, and 

 a rudimentary gill cover, as sturgeons. (2.) Plagiosto- 

 mata, having from five to seven external gill-openings and 

 no gill cover ; this latter order being subdivisible into 

 (a} sharks, wherein the trunk gradually passes into the 

 tail, and (b) skates and rays with a ventral gill-opening. 



If we look at the external appearance of a bony or 

 teleostean fish, as a carp or gurnard, we see a compara- 

 tively large head, attached to the trunk without the inter- 

 vention of a neck ; the body tapers off towards the posterior 

 extremity, and the sides are somewhat flattened. As irre- 

 gularities would impede progress through the water, a fish's 



