Pentacrinus.
Fig. 1. Specimen of a recent Pentacrinus Caput Medusæ, from the Caribbean Sea.
The Lily-shaped animals (Crinoidea), so named from a fancied resemblance of some species when in a state of repose to a closed lily, may be compared to a Feather-star (Comatula) fixed to a jointed column, with its mouth upwards; the base of the stem being attached to the rock by root-like processes. The only known living genus inhabits the seas of the West Indies, and the specimen figured represents the body (or upper part of the animal), with a considerable portion of the stem remaining attached. The Crinoidea are divided into two groups; Encrinites, having the ossicula (little bones) of the stem rounded, and Pentacrinites, in which the ossicula of the column are pentagonal, or angular. The Crinoidea are characterized by having a fixed base, a column or stem composed of numerous separate articulated pieces of a solid calcareous substance, supporting on its summit a vase, or receptacle, formed by a series of closely adjusted plates, which contain the body, or viscera. The upper part of the receptacle is covered by a plated integument, on one side of which an aperture or mouth is placed. From the upper margin proceed five articulated tentacula or arms, which subdivide into branches that in some species are very numerous and of extreme tenuity. On the inside, the arms are beset with articulated cirri or feelers. The joints composing the column are perforated by a central opening; there are also side-arms, that radiate from the column in groups of five at different points. When the animal is alive, the skeleton is covered by a soft integument, as in the star-fishes, and the arms spread out and expand, forming a net, by which living prey is captured and conveyed to the mouth by the tentacula, in the same manner as in the fresh-water polype or Hydra.
The fossil remains of Crinoidea consist of the ossicula of the column, arms, and tentacula; of the plates of the vase, or receptacle; and of the peduncle, or base of attachment. This family of Radiaria, though now of such excessive rarity, swarmed in the seas that deposited the ancient secondary strata; whole mountain chains and extensive tracts of country are composed of strata almost entirely made up of their fossil remains.[38] The number and species of genera is very great.
[38] Wonders of Geology, vol. ii. p. 645. Medals of Creation, p. 312.
Fig. 2, is a remarkably beautiful specimen of the receptacle of a Pentacrinite from Gloucestershire, showing the arms introverted, as if the animal had suddenly perished while in the act of closing over its prey; the stem is wanting.
Fig. 3. A spongite (Chenendopora subplana, of Michelin) from the greensand of the Vale of Pewsey, in Wiltshire.
Plate XLVII.