Later we find that
it has lungs, and a heart with four chambers instead of only two, as
in fish. The vertebrae of its backbone are not biconcave, but flat in
front and behind. And, finally, we discover that it suckles its
young. It, too, is in all its deep-seated characteristics a mammal.
It is fish-like only in characteristics which it might easily have
acquired in adaptation to its aquatic life. And there are other
aquatic mammals, like the seals, in which these characteristics are
much less marked. Their adaptation has evidently not gone so far.
Now the first attempts resulted in artificial classifications, much
like our grouping of bats with birds and whales with fish. All
animals, like coral animals and starfishes, whose similar parts were
arranged in lines radiating from a centre, were united as radiates,
however much they might differ in internal structure and grade of
organization. But this radiate structure proved again to be largely
a matter of adaptation.
Practically all animals having a heavy calcareous shell were grouped
with the snails and oysters as mollusks. But the barnacle did not
fit well with other mollusks. Its shell was entirely different. It
had several pairs of legs; and no mollusk has legs. The barnacle is
evidently a sessile crab or better crustacean. Its molluscan
characteristics were only skin-deep, evidently an adaptation to a
mode of life like that of mollusks.
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