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DIVISION I. ICPITHYOPSIDA. 



CHAPTER LIV. 

 CLASS L PISCES. 



THE first class of the Vertebrata is that of the Fishes (Pisces), 

 which may be broadly defined as including Vertebrate animals 

 which are provided with gills throughout the whole of life ; the 

 heart, when present, consists (except in Lepidosireti) of a single 

 auricle and a single ventricle ; the blood is cold ; the limbs, when 

 present, are in the form of fins, or expansions of the integument ; 

 and there is neither an amnion nor allantois in the embryo, unless 

 the latter is represented by the urinary bladder. 



In form, Fishes are adapted for rapid locomotion in water, 

 the shape of the body being such as to give rise to the least 



possible friction in swimming. To 

 this end also, as well as for purposes 

 of defence, the body is usually 

 enveloped with a coating of scales 

 developed in the inferior or dermal 

 layer of the skin. The more impor- 

 tant modifications in the form of 

 these dermal scales are as follows : 



I. Cycloid scales (fig. 1 6 1, a], con- 

 sisting of thin, flexible, horny 

 scales, circular or elliptical in shape, 

 and having a more or less com- 

 pletely smooth outline. These are 

 the scales which are characteristic 

 of most of the ordinary bony fishes. 



II. Ctenoid scales (fig. 161, b], 



olcr, rnrmisHncr of thin hnrriv nlatps 

 aiSO Consisting OI HUD llOmy piatCS, 



scale but having their posterior margins 

 Dinged with spines, or cut into 

 comb-like projections. III. Ganoid 



scales, composed of an inferior layer composed of bone, covered 

 by a superficial layer of hard polished enamel (the so-called 



Fig. 161. Scales of different fishes, 

 a Cycloid scale (Pike) ; ^Ctenoid 



scale (Perch) ; c Piacoid 



