340 



MANUAL OF ZOOLOGY. 



DIVISION L ICHTHYOPSIDA. 



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 (with one exception) of a single au- 

 ricle 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 important modifications 

 in the form of these dermal scales 

 are as follows : I. Cycloid scales (fig. 

 124, a), consisting of thin, flexible, 

 horny scales, circular or elliptical 

 in shape, and having a more or less 

 completely smooth outline. These 

 are the scales which are character- 

 istic of most of the ordinary bony 

 fishes. II. Ctenoid scales (fig. 124, b\ 

 also consisting of thin horny plates, 

 but having their posterior margins 

 fringed with spines, or cut into 



Fig. 124. Scales of different fishes. COmb-Hke projections. III. Gail- 



iSft^T^Jrid'SS ^scales, composed of an inferior 



(Thornback); d Ganoid scales layer Composed of bone, COVCred 



by a superficial layer of hard po- 

 lished enamel (the so-called " ganoine "). These scales (fig. 



