INTRODUCTION 
fertilising purposes. ‘This accomplished, no further 
heed is paid to what happens, and fishes seem to resemble 
insects in this respect. Exception must be made, how- 
ever, in the case of that plucky little “ soldier,” the 
Stickleback, who not only builds a nest, but jealously 
guards it and his wife against invasion. It is a notable 
exception of great interest, and will be referred to later 
when the life-story of this favourite species is being 
unfolded. 
Of the external appearance of a fish it need only be said 
here that, whilst some have large scales—like the Roach 
and Rudd—others bear very small ones—like the bottom- 
loving Gudgeon—and others again—like the Eel and 
Tench—are practically scaleless as compared with their 
heavily-plated brethren. The tapering body, that is, 
having the head and hind quarters narrower and more 
torpedo-shaped than the deeper central parts, is well 
known, and its use in permitting the owner to propel 
itself through the water with the least resistance is 
equally familiar. Not that all our fishes are thus shaped, 
for the Eel and Lamprey are snake-like in appearance, 
and certain marine species are far different in form, 
but the typical example will serve our purpose. The 
body bears upon it fins equivalent to the limbs possessed 
by a quadruped, or the wings and legs of a bird. If 
we take that voracious and social fresh-water inhabitant, 
the Perch, as a basis for explanation, we find that along 
the back it has two fins, the one in front being called the 
first dorsal, and the one immediately behind is the second 
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