109 



CHAPTER XXX. 



FISHES. 



The first class of the Vcrtebrata 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 {with few exceptions) of a single- 

 auricle and a single ventricle ; the hlood. is cold ; the limhs, 

 %vhen present, are in the form of fins, or expansions of the 

 integument. 



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 wdth a 

 coating of scales developed in the inferior or dermal layer of 

 the skin. The more important modifications in the form of 

 these dermal scales are as folio w^s: 1. Cycloid scales (fig. 484), 



Fig. 484.— Cycloid .scale. 



Fig. 485. — Ctenoid scale. 



Fig. 486.— Ganoid scale. 



consisting of thin, flexible, horny scales, circular or elliptical 

 in shape, and having a more or less completely smooth out- 

 line. These are the scales which are characteristic of most 

 of the ordinary bony fishes. 2. Ctenoid scales (fig. 485), 



