PISCES. 



207 



tubercles, or plates, scattered through the skin, and sometimes 

 armed with projecting spines. 



4. Ganoid scales (<f,) composed of a layer of true bone, 

 covered by a layer of hard polished enamel. These scales are 

 usually much thicker and larger than the ordinary scales ; they 

 are often oblong or rhomboidal in shape ; they are often con- 

 nected together by little processes ; 

 and they generally are in contact by 

 their edges, but rarely overlap one 

 another. In most fishes there is 

 also to be observed a line of peculiar 

 scales, forming what is called the 

 "lateral line." Each of the scales 

 of this line is perforated by a minute 

 tube, which leads into a longitudinal 

 canal, believed to secrete the mucus 

 with which the general surface is 

 lubricated, or to have some sensory 

 function. 



As regards the true internal 

 skeleton, fishes differ very widely 

 from one another, but the skeleton 

 is so complicated that only a few of 

 the most important points can be 

 mentioned here. In one fish — the 

 Lancelet — there can hardly be said 



to be any true skeleton, the vertebral column being repre- 

 sented permanently by the semi-gelatinous notochord (Fig. 

 105). In others, such as the Lampreys, Sturgeons, and Rays, 

 the skeleton remains permanently in the condition of gristle 

 (cartilage) ; in others it is partially cartilaginous and partially 

 ossified ; and, lastly, in most modem fishes it is completely 

 converted into bone. The vertebral column in a bony fish 

 consists of a number of vertebrae which are hollow or cup- 

 shaped at both ends (bi-concave or " amphicoelous "), the cup- 

 like margins being united together by ligaments^ The cavities 

 formed by the apposition of the vertebrae are filled with the 

 gelatinous remains of the notochord. This gelatinous elastic 

 substance acts as a ball-and-socket joint between the vertel^rse, 

 thus giving the whole spine the extreme flexibility which is 

 essential to animals living in a watery medium. The entire 

 spinal column is divisible into no more than two distinct 

 regions, an abdominal and a caudal. The ribs are attached 

 to the transverse processes or to the bodies of the abdominal 



Fig. 100. 



■Scales of different Fishes. 

 a Cycloid scale (Pike) ; & Ctenoid 

 scale (Perch); c Placoid scale 

 (Thornback) ; d Ganoid scale (Pa- 

 Iceoniscm). 



