MECHANISM OF SIDE-AK.MS* 



Side-Ar?ns. 



The Side-Arms become gradually smaller towards the 

 upper extremity of the column. In the P. Briareus (PI. 52, 

 Fig. 3. and PL 53, Fig. 1. and 3.) these amount to nearly a 

 Thousand in number.* The numerous side-arms of the 

 Briarean Pentacrinite, when expanded, would act as aux- 

 iliary nets to retain the prey of the animal, and also serve 

 as hold-fasts to assist it in adhering to the bottom, or to ex- 

 traneous 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 tlie specinien, PI. 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 this thousand side-arms, each having from fifty to a hun- 

 dred joints, PI. 53, Fig-. 14. The number of joints 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 

 rcaembling a mortice and tenon ; and the form both of the articulating sur- 

 faces and of the bone itself, varies so as to give more universal motion 

 as they advance towards the small extremity of the arm. See PI. 53, Fig. 

 14. a. b. 



In all this delicate mechanism which pervades ever}' individual side- 

 arm, we see provision for the double purpose of attaching itself to extrane- 

 ous bodies, and apprehending its prey. Five of these arms are set oif 

 from each of the largest joints of the vertebral column. At PI. 53. Fig. 

 7. a. we see the bases, or first joints of these side-arms articulating with 

 the larger vertebrce, and inclined alternately to the right and left, for the 

 purpose of occupying their position most advantageously for motion, witJ>- 

 out interfering with each other, or with the flexure of the vertebral column. 



In the recent Pentacrinus Caput Medusce (PI. 52, Fig, 1.) the side-arnii* 

 (D.) are dispersed at distant intervals along the columa. 



