The "column" is the stem
by which the animal is attached permanently to the bottom of the
sea; and it is composed of numerous separate plates, so jointed
together that whilst the amount of movement between any two pieces
must be very limited, the entire column acquires more or less
flexibility, allowing the organism as a whole to wave backwards and
forwards on its stalk. Into the exquisite _minutioe_ of structure
by which the innumerable parts entering into the composition
of a single Crinoid are adapted for their proper purposes in
the economy of the animal, it is impossible to enter here. No
period, as before said, has yielded examples of greater beauty
than the Upper Silurian, the principal genera represented being
_Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus,
Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus,
Glyptocrinus, Crotalocrinus_, and _Edriocrinus_.
[Illustration: Fig 62.--Upper Silurian Crinoids. a, Calyx and
arms of _Eucalyptocrinus polydactylus_, Wenlock Limestone; b,
_Ichthyocrinus loevis_, Niagara Limestone, America; c, _Taxocrinus
tuberculatus_, Wenlock Limestone. (After M'Coy and Hall.)]
[Illustration: Fig. 63.--_Planolites vulgaris_, the filled-up
burrows of a marine worm. Upper Silurian (Clinton Group), Canada.
(Original.)]
The tracks and burrows of _Annelides_ are as abundant in the
Upper Silurian strata as in older deposits, and have just as
commonly been regarded as plants.
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