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 embedded (Zuryaleac). When plated, the covering consists typically 
of four rows of calcareous plates, known as the upper, lower, and side arm- 
plates (Scutella dorsalia, ventralia, and lateralia), (Fig. 326, 4). The lateral 
or adambulacral plates usually carry rows of mobile spines. 
The greater part of the interior of the arms is occupied by a linear series of 
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 interbrachial 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 

y Fic. 326. 
A, Vertical section of an Ophiuran arm. w, Vertebral ossicle ; «, Ambulacral vessel, with side-branches lead- 
ing into the tube feet ; b, Blood-vessel; n, Nerve-cord ; v, Ventral or lower arm-plate ; J, Side-plates ; d, Dorsal 
plate. B, Vertebral ossicle, seen from the inward side, with surrounding arm-plates. C, Row of vertebral 
ossicles viewed from the side, and slightly enlarged ; 2, Apertures where the branches of the ambulacral vessel 
enter and emerge from the arm-bones ; y, Depressions for the insertion of intravertebral muscles. D, Mouth- 
frame of an Ophiuran, with the proximal vertebral ossicles. The heavy lines 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 lursae. 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 
