228 VERTEBRATE ANIMALS. 
tubercles, or plates, scattered through the skin, and sometimes 
armed with projecting spines, 
4. Ganotd scales (d@), 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 ordi- 
nary scales ; they are often oblong 
or rhomboidal in shape; they are 
often connected 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, 
Fig. 122.—Scales of different Fishes. believed to secrete the mucus with 
@ Cycloid scale (Pike); 4 Ctenoid : 
scale (Perch); c Placoid scale Which the general surface is lu- 
(Thornback) ; @ Ganoid scale bricated, or to have some sensory 
(Paleoniscus). 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 represented permanently 
by the semi-gelatinous notochord (fig. 127). In others, such as 
the Lampreys, Sturgeons, and Rays, the skeleton remains per- 
manently in the condition of gristle (cartilage); in others, it is 
partially cartilaginous and partially ossified ; and, lastly, in 
most modern fishes it is completely converted into bone. The 
vertebral column in a bony fish consists of a number of vertebrze 
which are hollow or cup-shaped at: both ends (biconcave or 
“amphiccelous”), the cup-like margins being united together by 
ligaments. The cavities formed by the apposition of the ver- 
tebre are filled with the gelatinous remains of the notochord. 
This gelatinous elastic substance acts as a ball-and-socket joint 
between the vertebrze, thus giving the whole spine the extreme 
flexibility which is essential to animals living in a watery me- 
dium, The entire spinal column is divisible into no more than 
