am. 



viii EGHINOnEBMATA— MORPHOLOGY OF SKELETON 357 



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 (Ophiohelus, Fig. 313) in which 

 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. 



Ml if 



only small spaces are to be 



found, which dorsally contain 



continuations of the body K^fy?' 



cavity of the disc, while ven- 



trally they contain the radial 



water vascular trunk, the 



radial nerve cord, the epineural 



canal, and the pseudohtemal 



vessel. The lateral branches 



of the radial vessels of the 



water vascular system, before fig. 313.— OpWohelus umbeUa, Lym. a macerated 



enterinjy each tube-foot pass joint from near the tip of an arm, from the dorsal side 



, ^ -I ■^' ^.r^ (after Lyman), ds. Dorsal; ss, lateral .shield; cnii, 



tnrough, on eacn Siae, tne ambulacral ossicles ; apa, 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 Ophiuridae, whereas in the Euryalidce 

 the arms can be completely rolled up orally {cf. 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 AstropMjtidcc (Euryalidce) and the 

 Ophiomyxidce, and the arms are, in these animals, covered hy 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 Asteroidea, the terminal 



