[90 THE INHABITANTS OF THE SEA. 
earth. Those fishes which are destined to live at the bottom of 
the sea or to conceal themselves in the mud, such as eels and 
skates, have either no air-bladder or a very small one—for 
economical Nature gives none of her creatures any organ that 
would be useless to them. Even the slimy glutinous matter 
which is secreted from the pores of most fishes, and lubricates 
their bodies, assists them in gliding through the waters, so that 
no means have been neglected to promote the rapidity of their 
movements. 
The skin of fishes is but seldom naked ; in most species it is 
covered with scales, that sometimes appear in the form of 
osseous plates, as in the ostracions, or project into formidable 
prickles, as in the poreupine-fish, but generally offer the aspect 
of thin laminz, overlapping each other like the tiles of a roof, 
and embedded, like our nails, in furrows of the skin. In nearly 
all the existing fishes, the scales are flexible and generally either 
of a more or less circular form (cycloid), as in the salmon, 
herring, roach, &c., or provided with comb-like teeth projecting 
from the posterior margin (ctenoid), as in the sole, perch, pike, 
&e.; while the majority of fossil fishes were decked with hard 
bony scales, either rhomboidal in their form, of a highly 
polished surface, as in our sturgeons (ganoid), and arranged in 
regular rows, the posterior edges of each slightly overlapping 
the anterior ones of the next, so as to form a very complete 
defensive armour to the body; or irregular in their shape and 
SAY Wily eres 
TPAD SOCK, | VW). OSES 
Wide Fb his duit NO 
Portion of Skin of Sole highly magnified, 
‘a 
separately imbedded in the skin (p/acozd), as in the sharks and 
rays of the present day. 
The scales of almost any fish afford admirable subjects for 
microscopic observation, but more particularly those of the 
ctenoid kind, which exhibit a brilliancy of reflected light, and a 
