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. The old artificial systems were
based too much on merely external characteristics, the results of
adaptation. When the internal anatomy had been thoroughly studied
their groups had to be rearranged.

Reptiles and amphibia were at first united in one class because of
their resemblance in external form. Our common salamanders look so
much like lizards that they generally pass by this name. But the
young salamander, like all amphibia, breathes by gills, its skeleton
differs greatly from, and is far weaker than, that of the lizard,
and there are important differences in the circulatory and other
systems. Moreover, practically all amphibia differ from all reptiles
in these respects. Evidently the fact that the alligator and many
snakes and turtles (of which neither the young nor the embryos ever
breathe by gills) live almost entirely in the water, is no better
reason for classifying these with amphibia than to call a whale a
fish, and not a mammal, because of its form and aquatic life.