DIVISION I. ICHTHYOPSIDA. 



CHAPTER LIV. 



CLASS I. — PISCES. 



The first class of the Vertebrata is that of the Fishes (JPisces), 

 which may be broadly defined as including Vertebrate animals 

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

 heart, when present, consists {with one exception) 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, tmless 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 en- 

 veloped with a coating of scales 

 developed in the inferior or dermal 

 layer of the skin. The more im- 

 portant modifications in the form 

 of these dermal scales are as fol- 

 lows: I. C^ir/wV scales (fig. 132, «), 

 consisting of thin, flexible, -homy 

 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. 132, b\ 

 '''l-gci^d'Sli.fpnlf'rfcfctTd also consisting of thin horny plates, 

 scale {Perch); c piacoid scale but having their posterior margins 



(Thomback): tf Ganoid scales (/'a- j- • ■, ■,! . r 



laonUcus). frmged With spmes, or cut mto 



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 



