But the struggle was in vain, as far as the
supremacy of the animal kingdom was concerned. Their ancestors had
taken a course which rendered it impossible for their descendants to
reach the goal. Their progress became ever slower. They were
entirely and hopelessly beaten by the vertebrates. They struggled
hard, but too late.
The history of mollusks is full of interest. They show clearly how
intimately nervous development is connected with the use of the
locomotive organs. The snail crept, and slightly increased its
nervous system and sense-organs. The clam almost lost them in
connection with its stationary life. The cephalopods were
exceedingly active, developed, therefore, keen sense-organs and a
very large and complicated supra-oesophagal ganglion, which we
might almost call a brain.
The articulate series consists of two groups of animals. The higher
group includes the crabs, spiders, thousand-legs, and finally the
insects, and forms the kingdom of arthropoda. The lower members are
still usually reckoned as worms, and are included under the
annelids. Of these our common earthworm is a good example, and near
them belong the leeches. But the marine annelids, of which nereis,
or a clam-worm, is a good example, are more typical. They are often
quite large, a foot or even more in length.
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