72 



CONNECTICUT GEOL. AND NAT. HIST. SURVEY. 



[Bull. 



central cavity occupied by the digestive and reproductive organs, 

 the body cavity in the arm of the ophiuran is represented only 

 by a narrow canal (Fig. 10). Each arm is made up of a great 

 number of similar segments. The bulk of the arm consists of 

 a series of solid, cylindrical disks, called vertebrae, which are 

 arranged end to end in a single row, and taper gradually to the 

 tip of the arm. 



Fig. io. Diagram of transverse section of 

 arm of an ophiuran. C, branch of ccelom; L, 

 lateral arm plate ; M, muscular attachment of 

 vertebra; N, radial nerve; S, arm spines; T, 

 tentacle ; T S, tentacle scales ; U, upper arm 

 plate ; U N, under arm plate ; V, vertebra ; IV, 

 radial canal of water- vascular system. (Modified 

 from MacBride.) 



Each vertebra, in all our native species, articulates with its 

 neighbor by a ball-and-socket joint, the concavity on the proximal 

 side interlocking with a corresponding projection on the distal 

 side of its neighbor. 



The vertebrae are joined to each other by four strong muscles 

 (Fig. io), attached symmetrically to their faces. The contrac- 

 tion of the two upper muscles of a series of segments bends the 

 arm upward; a similar contraction of the lateral or lower pair 

 bends the arm sidewise or towards the oral surface as the case 

 may be. By suitable combinations of such muscular contractions 

 the arms can be moved rapidly and bent about in all possible 

 directions, with the one exception that they are so articulated 

 that they cannot be coiled in a spiral above the disk. 



On the aboral side of each vertebra (Fig. io) is a shallow 

 groove which contains the slender extension of the body cavity, 



