The second class of Eleutherozoa are familiarly known as "Brittle Stars," on account of their tendency, when seized, to escape by snapping off an arm, although this habit is by no means confined to them, but is shared in a marked degree by many Asteroidea, such as Luidia, for instance. Like the Asteroidea, they are "starfish," that is to say, they consist of a disc and of arms radiating from it; but the scientific name Ophiuroidea really expresses the great dominating feature of their organisation. Literally it signifies "Snake-tail" (ὄφις, snake; οὐρά, tail), and thus vividly describes the wriggling, writhing movements of the long thin arms, by means of which the Ophiuroid climbs in and out of the crevices between the stones and gravel in which it lives. This feature, viz. the effecting of movement by means of muscular jerks of the arms, instead of by the slow protrusion and retraction of the tube-feet, is the key to the understanding of most of the points wherein the Brittle Stars differ from the true Starfish.
Asteroidea and Ophiuroidea agree in the common ground-plan of their structure, that is, they both possess arms; but the most obvious difference in their outer appearance is that whereas in Asteroidea the arms merge insensibly into the disc, in Ophiuroidea the disc is circular in outline and is sharply marked off from the arms. Closer inspection shows that in the Ophiuroid the arms are continued inwards along grooves, which run on the under surface of the disc, and that they finally coalesce to form a buccal framework surrounding the mouth. In the very young Ophiuroid the arms melt into a small central disc, as in the Starfish, but the disc of the adult is made up of a series of interradial dorsal outgrowths which meet one another above the arms.
Fig. 207.—Aboral view of Ophiothrix fragilis. × 1. r, Radial plate.
Fig. 208.—Oral view of the disc of Ophiothrix fragilis. g.b, Opening of the genital bursa; m.p, madreporite; pod, podia; t.p, tooth-papillae; v.p, ventral plates of the arms. × 1.
One of the commonest British Ophiuroids is Ophiothrix fragilis (Figs. 207, 208), which is found in swarms in shallow water off the west coast of England and Scotland. We may therefore select it as the type, and, since the arm is the most characteristic organ of an Ophiuroid, we may commence by studying it. Speaking generally, an Ophiuroid either drags itself forward by two arms and pushes itself by the other three (Fig. 207),[[458]] or else it drags itself by one and pushes with the other four (Fig. 217). The arms during this process are bent into characteristic curves, by the straightening of which in the posterior arms the animal is pushed onwards, whilst the intensification of these curves in the anterior arms causes the animal to be dragged forwards. The grip of the arm on the substratum is chiefly in the distal portion of the curve. The alteration of the curvature is due to the contraction of the muscles on one side of the arms. There is no ambulacral groove such as is found on the under side of the arms of all Asteroidea, for the arm is completely ensheathed by four series of plates, an upper row of dorsal plates, an under row of ventral plates, and two lateral rows of lateral plates. The last named, which in all probability correspond to the adambulacral plates of Starfish, bear each a transverse row of seven spines with roughened surfaces; these enable the animal to get a grip on the substratum over which it moves. The podia in Ophiuroidea are termed "tentacles"; they are totally devoid of suckers, being simple conical papillae used as sense-organs, and are of little, if any, service in locomotion. They issue from openings called "tentacle-pores" situated between the edges of the ventral and lateral plates, guarded each by a valve-like plate called the "tentacle-scale." In Ophiothrix they are covered with sense-organs, each consisting of a hillock-like elevation of the ectoderm, in which are cells carrying long stiff sense-hairs. In most Ophiuroids such organs are not present, though abundant scattered sense-cells occur, and the outer surface of the tube-feet and the lining of certain pockets called "genital bursae" (Fig. 208, g.b) are the only portions of the surface where the ectoderm persists. Everywhere else, although present in the young, it disappears, leaving as remnants a few nuclei here and there attached to the under side of the cuticle.[[459]]
Fig. 209.—Diagrammatic transverse section of the arm of an Ophiuroid. coe, Dorsal coelomic canal; ect, ectoderm covering the tube-foot; ep, epineural canal; gang.p, pedal ganglion; L, nerve-cord; musc, longitudinal muscles attaching one vertebra to the next; nerv.rad, radial nerve-cord; perih, radial perihaemal canal; pod, podium (tube-foot); sp, lateral spines; w.v.r, radial water-vascular canal.
The greater part of the section of the arm is occupied by a disc-like ossicle called the "vertebra." Each vertebra articulates with its predecessor and successor by cup-and-ball joints, and it is connected to each of them by four powerful longitudinal muscles. Above, its outline is notched by a groove, in which lies an extension of the coelom of the disc (Fig. 209, coe), but contains no outgrowth of the alimentary canal, as is the case in Asteroidea. The vertebra is also grooved below, and in this lower groove are contained the radial water-vascular canal (Fig. 209, w.v.r), and below it perihaemal canals as in Asteroidea; below this again the radial nerve-cord (L), and beneath this again a canal called the "epineural canal" (ep), which represents the missing ambulacral groove. This canal in the very young Brittle Star is an open groove, but becomes closed by the approximation of its edges. The vertebra, which has a double origin, represents a pair of fused ambulacral ossicles. In Ophiohelus these are only slightly adherent to one another (Fig. 216).