CLASS II 



OPHIUEOIDEA 



253 



do not contain diverticula of the alimentary canal nor of the sexual organs. 

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

 or protected by a coriaceous skin, in which minute granules and scales are 

 embedded. When plated, the covering consists typically of four rows of 

 calcareous plates, known as the upper, lower and side arm-plates (Fig. 358, A). 

 The lateral or adamhulacral plates usually carry rows of mobile spines. 



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

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

 which is made up of two, or possibly of four, ambulacral pieces soldered side 

 by side (Fig. 358, B, C). The halves of the first two vertebral disks ai"e 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 Avhich project from the centres of both surfaces, 

 the interspaces being filled with muscles. The entire series is incised inferiorly 

 along the median line for the reception of the radial water-tube, beneath 

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

 the integument. The radial ambulacral vessel (water-tube) gives off" a pair of 

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



^^<^-^. 



Fig. 359. 



Portion of central disk of Ophiura viewed 

 from the dorsal side, a, Radial shields ; 

 h, Upper arm-plates ; c, Side arm-plates. 



Fig. 360. 



Portion of central disk of Ophiura 

 viewed from the ventral side, a, Mouth 

 shield ; h, Side mouth shield ; c, Jaws 

 bearing papillae ; g, Genital slits ; h, Side 

 arm-plates ; (', Pores for the emission of 

 the tube -feet, surrounded with tentacle 

 scales ; fc, Spines. 



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, star-shaped aperture of 

 the mouth (Fig. 363), 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 fissures (genital slits), which skirt the arm bases inferiorly, 

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

 discontinuous (Ophioderma), appearing as two slits, one behind the other ; and in 

 some fossil forms they are represented by rows of pores. 



The integument covering the entire upper surface of the disk and the 



'interbrachial area on the ventral side is frequently beset with calcareous 



plates ; but this scale coat may be covered in turn with a thick skin, or bear 



spines or granules. A large central plate is sometimes recognisable on the 



dorsal aspect of the disk, together with five pairs of plates, which, from their 



