22& 



VElfTEltrfATE ANIMALS. 



ICHTIIYOPSIDA. 



CHAPTER XXIII. 



CLASS I. PISCES. 



The fishes form the lowest chiss of the Vertebrata, and they may be 

 broadly defined as being Vertebrate animals provided icit/i gills, 

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

 heart, when present, con-nsts of a single auricle and ventricle (with 

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

 in the form of fins, or expansions 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 possiljle friction in swimming. 

 To this end, as well as for ])nrposes 

 of defej)ce, the body is generally 

 enveloped in a species of chain-mail 

 formed by overla]i|)ing .scales, to 

 Avliich bony plates, tubercles, and 

 s|)ines are sometimes added. Val- 

 uable characters can sometimes be 

 drawn from the nature of the scales; 

 and witli a view to this, the integu- 

 mentary aii|iendages of fishes have 

 been divided by Agassiz as follows 

 (fig. 104) :— 



1. t'l/cloid scales {<(), consisting of 

 tlun, rtexible, horny acale.s, which 

 iire circular or elliptical in .shape, 

 and have a smooth outline. These 

 scales occur in most of our common fishes ('-•.(/., the Pike). 



2. Ctenoid scales {b). — Tiiese resendile the cycloid scales in being 

 thin, flexil.ile, horny scales, but they are distinguished by having 

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



Fig. 104.— Scales of dillereut lisl 

 a Cycloid scale (Pike); b Ck-u 

 scalc(Pi'reh); c Pl.acoi.l scalf(Tlu) 

 Lack); d Placoid .scale nf Hhiin: 

 Guiiniil scales (Puhctnihrn^.) 



