VIII ECHINODERM AT A— MORPHOLOGY OF SKELETON 357 



apei- 



ajTb 



able name, since they play a part altogether similar to that of the 

 vertebrae of the axial skeleton in Vertebrate animals. In a large 

 majority of cases the two lateral portions of a vertebral ossicle are 

 fused in the median plane in such a way that no sutures are now to 

 be seen. These ossicles, however, arise ontogenetically as two, at first 

 entirely distinct, lateral pieces, which only fuse later. There are, 

 further, certain deep-sea Ophiuroidea {O'phiohdus, Fig. 313) in Avhicli 

 each vertebral ossicle consists, even in the adult, of two distinct slender 

 pieces, articulated one with the other. 



The vertebral ossicles fill up the greater part of the skeletal tube 

 formed by the dorsal, ventral, and lateral shields. Between them and 

 the tube, in dried skeletons, 

 only small spaces are to be 

 found, which dorsally contain 

 continuations of the body 

 cavity of the disc, while ven- 

 trally they contain the radial 

 water vascular trunk, the 

 radial nerve cord, the epineural 

 canal, and the pseudoha^mal 

 vessel. The lateral branches 

 of the radial vessels of the 



AVater vascular system, before Fig. 313.— OpWolielusumbella.Lym. a macerated 



entering each tube-foot, pass joint from near the tip of an arm, from the dorsal siile 



,1 1 V. ■ 1 f 1 (after Lyman), ds, Dorsal ; ss, lateral shield ; am, 



tnrOUgll, on eacn Siae, tne ambulacral ossicles ; f^ia, hook spines. 



substance of the vertebral 



ossicle of the corresponding segment, nearer the distal than the 

 proximal end of the ossicle. The consecutive vertebral ossicles of the 

 arms articulate one with another, and are connected by means of four 

 intervertebral muscles. The contraction of the two upper inter- 

 vertebral muscles brings about the upward curving, and the contrac- 

 tion of the two lower, the downward curving, of the arms. The 

 horizontal (lateral) movement is brought about by the contraction of 

 the upper and lower muscles of the same side. The vertical movement 

 of the arms is very slight in true Ophiuridse, whereas in the Euryalidce 

 the arms can be completely rolled up orally (c/. Fig. 246, p. 301). 



Small accessory plates may occur in addition to the dorsal shields. The super- 

 ficial brachial skeleton is much reduced in the AstrophytMcv (EuryuUd(c) and the 

 OphiomyxidcL', and the arms are, in these animals, covered by a soft integument, in 

 which only small skeletal pieces occur. In other forms the brachial skeleton is so 

 covered by an integument, often containing small embedded skeletal pieces, that it 

 is either partly or altogether invisible externally. 



At the distal end of each arm in the Ophiuroidea there is, as in 

 the Asteroidea, an unpaired median terminal, which surrounds the 

 tip of the radial water vascular trunk (the terminal tentacle) in the 

 form of a short skeletal ring. Since, in the AderovJea, the terminal 



