Star-Fish.
The upper tuberculated surface is shown, with some of the spines of the under surface projecting at the sides of the rays. At one of the angles between the rays, on the right side, is seen the eccentric calcareous plate, or madreporic tubercle, which indicates the existence of a bilateral symmetry.
This act of volition is surely remarkable enough in so simple an animal, which scarcely possesses the rudiments of a nervous system, but the simple mechanism by which the suckers are put into motion is still more wonderful. Each of these little organs is tubular, and connected with a globular vesicle filled with an aqueous fluid, and contained within the body of the star-fish immediately beneath the hole from which the sucker issues. When the animal wishes to protrude its feet, each vesicle forcibly contracts, and, propelling the fluid into the corresponding sucker, causes its extension; and, when it desires to withdraw them, a contraction of the suckers drives back the fluid into the expanding vesicles. The internal walls of the suckers and their vessels are furnished with vibratory cilia, and by this simple means a continual circulation of the fluid they contain goes on within them.
Lily-Encrinite.
Numerous species of star-fishes are so very common in our waters, that in many places the sea-bottom is literally paved with them. They likewise abounded in the primeval ocean, for deep beds of carboniferous limestone and vast strata of the triassic muschelkalk are often formed by the accumulation of little else than the skeletons of Encrinites and Pentacrinites, which, unlike the sea-stars which every storm drifts upon our shores, did not move about freely, but were affixed to a slender flexible stalk, composed of numerous calcareous joints connected together by a fleshy coat. The feathered bifurcated arms of the Crinoids are unprovided with suckers, which would have been perfectly useless to creatures not destined to pursue their game to any distance, but passively to receive the nutriment which the current of sea-water set in motion by their richly-ciliated pinnules conveys to the mouth. These beautiful creatures were formerly supposed to be nearly extinct, for up to within the last few years only two living stalked crinoids were known in the ocean of the present period, but the dredge has latterly brought up new and remarkably fine species from depths of more than 2000 fathoms, and there is every reason to believe that these animals still form an important element in the abyssal fauna.[S]
[S] See [page 420].
Portion of the Pentaorinus Briareus. (Fossil.)