■228 



VERTEBRATE ANIMALS. 



ICHTHYOPSIDA. 



CHAPTER XXIII. 



CLASS I. PISCES. 



The fislies form the lowest class of the Vertebrata, and they may be 

 broadly defined as being Vertebrate animals provided loith gills, 

 ■whereby they are enabled to breathe air dissolved in ■water ; the 

 heart, "when jJi'esent, consists of a single auricle and ventricle (with 

 the exception of the Mud-fishes) ; and the limbs, ■when present, are 

 in the form of Ji?is, or cipansio/is of the integument. 



In their external form, fishes are in most cases adapted for rapid 



locomotion in ■water, the shape of 

 the body being such as to cause the 

 least possible friction in s'wimming. 

 To this end, as well as for purposes 

 of defence, the body is generally 

 enveloped in a species of chain-mail 

 formed by overlapping scales, to 

 which bony plates, tubercles, and 

 spines are sometimes added. Val- 

 uable characters can sometimes be 

 drawn from the nature of the scales ; 

 and with a view to this, the integu- 

 mentary appendages of fishes have 

 been divided by Agassiz as follows 

 (fig. 164):- 



1. Cycloid scales (a), consisting of 

 thin, flexible, horny scales, which 

 are circular or elliptical in shape, 

 and have a smooth outline. These 

 scales occur in most of our common fishes {e.g., the Pike). 



2. Ctenoid scales (b). — These resemble the cycloid scales in being 

 thin, flexible, horny scales, but they are distinguished by having 

 their hinder margins cut into comb-like projections, or fringed with 



rig. 164.— Scales of different flslies. 

 a Cycloid scale (Pike); b Ctenoid 

 scale(Perch); c Flaccid scale(Tliorn- 

 back) ; d Placnid scale of Rhiiia ; e 

 Ganoid scales {Paloioniscus.) 



