CLASS I 



OPHIUROIDEA 



201 



The arms serve as -locomotive organs, and are either elegantly plated 

 (Ophiureae), or protected by a coriaceous skin, in which minute granules and 

 scales are embi-ddrd ( EwyaUae). When plated, the covering consists typically 

 of four rows of calcareous plates, known as the upper, lower, and side arm- 

 plates (Scutella dorxdia, nnfmlia, and lateralia), (Fig. 326, A). The lateral 

 or ,n /ambulacral plates usually carry rows of mobile spines. 



The greater part of the interior of the arms is occupied by a linear series ot 

 jointed, vertebrae-like sections called the vertebral ossicles or arm-bones, each of 

 which is made up of two ambulacral pieces soldered side by side (Fig. 326, 

 B, C). The halves of the first two vertebral disks are swung laterally into 

 the iriterbrachial space, being fused together to form the mouth angle. The 

 remainder of the arm-bones are movably articulated with one another by 

 means of bosses which project from the centres of both surfaces, the inter- 

 spaces being filled with muscles. The entire series is incised inferiorly along 

 the median line for the reception of the radiating water-tube, beneath which 



V Fi.:. 3215. 



A, Vertical section of an Ophiuran ami. w, Vertebral ossicle ; a, Ambulacral vessel, with side-branches lead- 

 ing into the tube feet ; b, Blood-vessel ; n, Nerve-cord ; i; Ventral or lower arm-plate ; I, Side-plates ; d, Dorsal 

 plate B, Vertebral ossicle, seen from the inward side, with BOrroondtng arm -plates. C, Row of vertebral 

 ossicles viewed from the side, and slightly .-nhn-.-d ; /. Apertures where the branches of the ambulacral vessel 

 enter and emerge from the arm-bones ; //, Depressions for the insertion of intravertebral muscles. D, Mouth- 

 frame of an Ophiuran, with the proximal vertebral ossicles. The heavy lilies bordering the arms represent the 

 bursal fissures ; the dark pentagon in the centre marks the course of the nerve-ring. 



runs the radiating blood-vessel and nerve cord, the whole being closed in by 

 the integument. The radiating ambulacral vessel gives off a pair of lateral 

 branches in each arm ossicle which pierce the bone itself, and supply the 

 tentacle-like tube feet with water. The tube feet are without either ampullae 

 or terminal suckers, and the orifice of the plates through which they protrude 

 is often protected by one or more minute tentacle scales (papillae ambulacrales), 

 which serve to cover the tentacles when they are drawn in. 



On the under side of the disk is seen the central, pentagonal aperture of 

 the mouth (Fig. 331), which leads into a large sac-like stomach. The latter 

 terminates blindly, there being no intestine. The body cavity also contains 

 the ambulacral, blood, and nerve rings, as well as the generative glands, whose 

 ducts open into folded pouches or bursae. The bursae are arranged in five 

 pairs, one to each interbrachial area, and communicate with the exterior by 

 means of slit-like bursal fissures, which skirt the arm bases inferiorly, and are 

 bounded by genital or bursal scales. Sometimes the fissures are discontinuous 



