Fig. 212.—Lateral view of mouth-frame of Ophiarachna incrassata. × 4. A1?, peristomial plate, possibly the half of the first vertebra; A2, the half of the second vertebra; A3, the third vertebra; F1, pores for pair of tentacles; gen, genital scale lying beside opening of genital bursa; musc, longitudinal muscles connecting vertebrae; n.r, groove for nerve-ring; p, tooth; p1, mouth-papilla; t, torus angularis. (After Ludwig.)
The mouth can be narrowed and the jaws forced inwards towards the centre by the simultaneous contraction of five muscles (musc. tr, Fig. 213) each, which unite the two halves of a jaw.
Turning now to the skeleton of the disc, we notice that dorsally it consists of a closely-fitting mosaic of small plates, which are usually concealed from view by a covering of minute spines. Opposite the insertion of each arm there are, however, a pair of large triangular plates ("radials"), which extend outwards to the periphery and strengthen it, much as the ribs do in an umbrella. These radial plates are always exposed, in Ophiothrix, even when the rest of the dorsal plates are concealed by spines. On the under surface there is a similar plating; but adjoining the jaws are five large, more or less rhomboidal, plates termed "scuta buccalia" (Fig. 211, J1), on one of which open the few madreporic pores which the animal possesses. Attached to the sides of the scuta buccalia are the "lateral mouth shields," which are in fact the adambulacral plates belonging to the second pair of ambulacral plates which form the main mass of the jaws. Further out, on the under side of the disc, there is, on each side of each arm, a long narrow slit—the opening of the genital bursa (Fig. 208, g.b), so that there are ten genital bursae. The "genital bursa" (Fig. 214) is a sac lined by ciliated ectoderm projecting into the interior of the disc. It is called genital because the openings of the genital organs are situated on its surface; its main function, however, is respiratory, the cilia bringing about a constant inward current of fresh sea-water, the oxygen contained in which diffuses through the thin wall of the sac into the coelomic fluid. The opening of the bursa is strengthened on its radial side by a rod-like ossicle, the "genital plate," and on its interradial side by an ossicle called the "genital scale" (Fig. 212, gen), and in Ophiothrix the outer end of the radial plate articulates with the outer end of the genital plate. Muscles connect the two plates running on either side of the articulation.
Observations on Ophiothrix[[460]] show that in this species at any rate the radial plates can be raised or lowered. When they are raised the centre of the disc is lifted into a cone and water is sucked into the genital bursae, whereas when they are lowered the bursae are compressed and water is expelled. This forced respiration appears to come into play when the supply of oxygen is getting scanty.
The alimentary canal of Ophiothrix is a simple flattened sac (Fig. 213). It is devoid of an anus and cannot be everted through the mouth. There is a horizontal pouch given off into each interradial lobe of the disc. The sac is attached to the dorsal wall of the coelom by numerous mesenteries, fibrous cords traversing the coelomic cavity and clothed on the outer side by coelomic epithelium. To the mouth-frame it is attached by a circular membrane, which we have reason for believing is a functionless remnant of the retractor muscles of the stomach of Asteroidea. In the young Asteroid there is a similar sheet of membrane, which later becomes resolved into the ten retractor bands.
The simple structure of the alimentary canal appears to be correlated with the exceedingly simple character of the food. Ophiothrix feeds on the most superficial layer of mud at the bottom of the sea. This deposit consists partly of microscopic Algae and partly of decaying organic matter, and is much more easily disposed of than the living animals on which the Starfish preys. The food is shovelled into the mouth by the first two or "buccal" pairs of tube-feet in each ray.
Fig. 213.—Longitudinal section through the disc of a young Ophiuroid passing along one arm and the middle of the opposite interradius. (Diagrammatised from an actual section of Amphiura squamata.) ab, Aboral sinus (dorsal in the radius, ventral in the interradius); ax, axial sinus; coe, dorsal coelomic canal of the arm; ep, epineural canal; gang.rad, ganglion of the radial nerve; gen.r, genital rachis contained in the aboral sinus; gen.st, genital stolon; mp, madreporic pore; musc.long, longitudinal muscle of the arm; musc.tr, transverse muscle uniting the two halves of each jaw; mv, madreporic vesicle; nerv.r, nerve-ring; p.c, pore-canal; perih, perihaemal canal; vert, vertebra; w.vr, radial water-vessel.
The water-vascular system has undergone a most interesting set of modifications, which can be explained by noticing the fact that the tube-feet have almost, if not quite, lost their locomotor function and are now used as tactile organs. The ampulla, or swollen inner end of the tube-foot, has disappeared, and the upper end of the organ is directly connected with the radial canal by means of a curved canal, which traverses the outermost flange of the vertebra, appearing on its surface in a groove on the outer side of the dorsal lateral knob on the distal side of the ossicle. As in Asteroidea there are valves, which regulate the entrance of fluid into the tube-foot. The stone-canal is a curved tube of simple circular section and excessively narrow bore which extends from the water-vascular ring downwards to the madreporite (Fig. 213, mp) situated on one of the scuta buccalia. The madreporite, in Ophiothrix as in most Brittle Stars, is an exceedingly rudimentary structure, consisting of one or two pores leading into as many pore-canals. From each interradius, except that in which the stone-canal lies, a large Polian vesicle hangs down from the water-vascular ring into the coelom.
We saw that in the Asteroid the ampulla was used like the bulb of a pipette to force the fluid in the tube-foot down into the tip, so as to press the sucker against the substratum. But when the tube-foot is used as a sense-organ, a few circular fibres around its upper end suffice to bring about all the extension that is needed. Since the extension is no longer a very vigorous act, the loss of fluid by transudation has probably been rendered insignificant, and hence the stone-canal and madreporite, whose function it is to repair the loss, have been reduced in size. The curious ventral curvature of the stone-canal is, however, due to another cause. In the very young Ophiuroid the madreporite is on the edge of the disc, and the stone-canal extends horizontally outwards; and in some Asteroidea there is a similar outward direction in its course. As development proceeds the dorsal interradial areas of the disc of the young Ophiuroid grow out into lobes, building up the conspicuous adult disc and forcing the madreporite, and with it the stone-canal, downwards towards the ventral surface.