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.
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