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Johnny Darter 
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A fish is a cold-blooded vertebrate ani- 
mal, which is fitted for aquatic life, 
breathing the air dissolved in the water, 
by means of structures called gills. It 
differs from the Amphibia by spending 
its whole life in the water and in never 
having its limbs provided with fingers or 
toes. 
At the bottom of the series are tuni- 
cates, lancelets and other primitive or 
degenerate forms, which hardly look like 
fishes. ‘The lowest which would adorn 
an aquarium is the lamprey, long, slim 
and eel-like, with a row of gill holes 
along the side, and with no paired fins or 
limbs, and no jaws, its big round mouth 
a sucking disc, provided with rasp-like 
teeth. I never knew any one to put a 
lamprey into an aquarium, but they 
would be very interesting. You can find 
them in the spring in most of the north- 
ern States, running up the brooks ‘to 
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! CLASSIFYING FISHES 
DAVID STARR JORDAN 
Leland Stanford Junior University 
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Boleosoma nigrum 
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spawn, fastening their lips to a stone and 
staying fastened till they wear out, a pic- 
turesque feature of the river-floor to 
which Thoreau once called attention. It 
is also a grim lesson of the reward of de- 
votion. 
Bigger fishes are divided into the class 
of sharks and the class of true fishes. 
Sharks do not thrive in a private aqua- 
rium. There isa little black sharklet that 
lives in deep bays in Japan, so far down 
that it is always in darkness. Its name is 
Etmopterus lucifer, and it is only a foot 
long. It has on each side of the belly a 
luminous patch, by which it sees its way. 
Dr. Schmidt, the Russian naturalist, put 
one in the aquarium, and then at night 
made a drawing of it by its own light. 
Of the true fishes, there are twenty or 
thirty large groups called orders. Each 
order contains from one to a hundred 
families. Each family is made up of 
