RADIARIES. 205 



adapted with equal skill and wisdom, the longi- 

 tudinal sutures favoring the proper flexure one 

 way, and the transverse ones allowing a curvature 

 in a contrary direction: and besides, by this 

 structure, as Mr. Gray has observed and De 

 Blainville intimates, the gradual increment of 

 the shell, by the deposition of fresh matter in all 

 these parts, is rendered easy. 



But the spines and suckers of these animals 

 are equally worthy of our notice and investiga- 

 tion .; the former as instruments of defence and 

 locomotion, and the latter as instruments of 

 locomotion, prehension, and respiration. I 

 mentioned the protuberances, large and small, 

 the latter usually planted round the former, 

 shaped like a breast with a central elevation 

 resembling the nipple, these afford a basis with 

 which the spines articulate, being united to it by 

 a membranous ligature or sac, so as to form a 

 kind of ball-and-socket articulation, working 

 upon these protuberances by means of the mem- 

 brane, the spines can assume every inclination 

 between vertical and horizontal, and may be 

 used both as motive and defensive organs. The 

 great zoological and physiological luminary of 

 Greece, Aristotle, observed of these animals that 

 they use their spines as legs for change of place, 1 

 and Reaumur, who paid particular attention to 



1 Hist. Anim. B. iv, c. 5, ad fin. 



