But another branch of the same group (Crustacea) has
meantime advanced, and it gradually supersedes the dwindling
Trilobites. Toward the close of the Silurian great scorpion-like
Crustaceans (Pterygotus, Eurypterus, etc.) make their appearance.
Their development is obscure, but it must be remembered that the
rocks only give the record of shore-life, and only a part of that
is as yet opened by geology. Some experts think that they were
developed in inland waters. Reaching sometimes a length of five
or six feet, with two large compound eyes and some smaller
eye-spots (ocelli), they must have been the giants of the
Silurian ocean until the great sharks and other fishes appeared.
The quaint stalked Echinoderm which also we noticed in the
Cambrian shallows has now evolved into a more handsome creature,
the sea-lily. The cup-shaped body is now composed of a large
number of limy plates, clothed with flesh; the arms are long,
tapering, symmetrical, and richly fringed; the stalk advances
higher and higher, until the flower-like animal sometimes waves
its feathery arms from the top of a flexible pedestal composed of
millions of tiny chalk disks.
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