46 1 



DIVISION I.ICHTHYOPSIDA. 



CHAPTER LI. 

 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 Dipnoi] 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 coat- 

 ing of scales developed in the 

 inferior or dermal layer of the 

 skin ; whereas the epidermis is 

 represented only by the slimy 

 mucus covering the exterior of 

 the animal. The more impor- 

 tant modifications in the form 

 of these dermal scales are as 

 follows : I. Cycloid scales (fig. 

 247, a), consisting of thin, 

 flexible, horny or bony scales, 



i IT ,_ i i Fig. 247. Scales of different fishes, a 



Circular _ Or elliptical m Shape, Cycloid scale : (Pike); b Ctenoid scale 



and having a more or less com- 

 pletely smooth outline. These 

 are the scales which are charac- 

 teristic of the most of the ordinary bony fishes. II. Ctenoid 

 scales (fig, 247, b\ also consisting of thin horny plates, but 



(Perch); c Placoid scale (Thornback) ; 

 d Placoid scale of Rhino,', e Ganoid 

 scales {Palceoniscus), 



