The
animal now boldly discards the protecting and confining shell, or
spreads over the outside of it, and becomes a "shell-fish" with
the shell inside. The octopus of our own time has advanced still
further, and become the most powerful of the invertebrates. The
Belemnite, as the Mesozoic cuttle-fish is called, attained so
large a size that the internal bone, or pen (the part generally
preserved), is sometimes two feet in length. The ink-bags of the
Belemnite also are sometimes preserved, and we see how it could
balk a pursuer by darkening the waters. It was a compensating
advantage for the loss of the shell.
In all the other classes of aquatic animals we find corresponding
advances. In the remaining Molluscs the higher or more effective
types are displacing the older. It is interesting to note that
the oyster is fully developed, and has a very large kindred, in
the Mesozoic seas. Among the Brachiopods the higher
sloping-shoulder type displaces the square-shoulder shells. In
the Crustacea the Trilobites and Eurypterids have entirely
disappeared; prawns and lobsters abound, and the earliest crab
makes its appearance in the English Jurassic rocks.
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