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Triassic seas
A rival to the Ammonite appeared in the Triassic seas, a
formidable forerunner of the cuttle-fish type of Cephalopod. 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. This sudden
arrival of a short-tailed Crustacean surprises us less when we
learn that the crab has a long tail in its embryonic form, but
the actual line of its descent is not clear. Among the
Echinoderms we find that the Cystids and Blastoids have gone, and
the sea-lilies reach their climax in beauty and organisation, to
dwindle and almost disappear in the last part of the Mesozoic.
One Jurassic sea-lily was found to have 600,000 distinct ossicles
in its petrified frame. The free-moving Echinoderms are now in
the ascendant, the sea-urchins being especially abundant. The
Corals are, as we saw, extremely abundant, and a higher type (the
Hexacoralla) is superseding the earlier and lower (Tetracoralla).
Finally, we find a continuous and conspicuous advance among the
fishes. At the close of the Triassic and during the Jurassic they
seem to undergo profound and comparatively rapid changes. The
reason will, perhaps, be apparent in the next chapter, when we
describe the gigantic reptiles which feed on them in the lakes
and shore-waters. A greater terror than the shark had appeared in
their environment. The Ganoids and Dipneusts dwindle, and give
birth to their few modern representatives. The sharks with
crushing teeth diminish in number, and the sharp-toothed modern
shark attains the supremacy in its class, and evolves into forms
far more terrible than any that we know to-day. Skates and rays
of a more or less modern type, and ancestral gar-pikes and
sturgeons, enter the arena. But the most interesting new
departure is the first appearance, in the Jurassic, of
bony-framed fishes (Teleosts). Their superiority in organisation
soon makes itself felt, and they enter upon the rapid evolution
which will, by the next period, give them the first place in the
fish world.
Over the whole Mesozoic world, therefore, we find advance and the
promise of greater advance. The Permian stress has selected the
fittest types to survive from the older order; the Jurassic
luxuriance is permitting a fresh and varied expansion of life, in
preparation for the next great annihilation of the less fit and
selection of the more fit. Life pauses before another leap. The
Mesozoic earth--to apply to it the phrase which a geologist has
given to its opening phase--welcomes the coming and speeds the
parting guest. In the depths of the ocean a new movement is
preparing, but we have yet to study the highest forms of Mesozoic
life before we come to the Cretaceous disturbances.
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