Side-Arms
The Side-Arms become gradually smaller towards the upper extremity of the column. In the P. Briareus (Pl. 52, Fig. 3. and Pl. 53, Fig. 1. and 3.) these amount to nearly a thousand in number.[1] The numerous side-arms of the Briarean Pentacrinite, when expanded, would act as auxiliary nets so retain the prey of the animal, and also serve as hold-fasts to assist it in adhering to the bottom, or to extraneous bodies. In agitated water they would close and fold themselves along the column, in a position which would expose the least possible surface to the element, and, together with the column and arms, would yield to the direction of the current.
- ↑ If we suppose the lower portion of the specimen, Pl. 53, Fig; 2. a. to be united to the upper portion of the fractured stem, Fig. 3, we shall form a correct idea of the manner in which the column of this animal was surrounded with its thousand side-arms, each having from fifty to a hundred joints, Pl. 53, Fig. 14. The number of joint: in the side-arms gradually diminishes towards the top of the vertebral column; but as one of the lowest and largest (Pl. 53, Fig. 14.) contains more than a hundred, we shall be much below the reality in reckoning fifty as their average number.
Each of these joints articulates with the adjacent joint, by processes resembling a mortice and tenon; and the form both of the articulating surfaces and of the bone itself] varies so as to give more universal motion as they advance towards the small extremity of the arm. See Pl. 53, Fig. 14. a. b.
In all this delicate mechanism which pervades every individual side-arm, we see provision for the double purpose of attaching itself to extraneous bodies, and apprehending its prey. Five of these arms are set off from each of the largest joints of the vertebral column. At Pl. 53. Fig. 7. a. we see the bases, or first joints of these side-arms articulating with the larger vertebræ, and inclined alternately to the right and left, for the purpose of occupying their position most advantageously for motion, without interfering with each other, or with the flexure of the vertebral column.
In the recent Pentacrinus Caput Medusa (Pl. 52, Fig. 1.) the sidea-arms (D.) are dispersed at distant intervals along the column.
