Echinozoa with a disc-like body and slender arms sharply marked off from it; arms plated; no ambulacral groove; madreporite on actinal surface; usually fused to an oral plate; tube-feet pointed, lateral in position; no anus; genital bursae at base and sides of arms, interradial.

The character of the integument, the number and arrangement of the plates on the body are subject to much variation. The extent to which the apical system retains the typical arrangement, or is obliterated by subsequently formed plates, differs much even within the limits of the same genus. When it is traceable, there is a dorso-central plate surrounded either (i) by a ring of five radials which typically appear before the remaining apical plates; or (2) by a ring of five basals (Ophiomitra exigud); or (3) by basals and radials with or without (4) superadded under-basals. There is often a circle of five primary interradials (inter-brachials), each plate touching a basal and thus separating the radials from one another. Second, third, etc. interradials carry on the line and abut on the actinal surface against the five large orals known as buccal shields. These shields are as a rule very distinct, and are absent only in the sub-class Euryalida with the exception of Trichaster. The integument of the disc is soft in the Ophiomyxidae. Calcareous granules and spines may cover the outer surface of the plates. In the Euryalida plates are absent, and granulations cover the surface and frequently bear attached spines.

It is the only group of Ophiuroidea in which structures resembling pedicellariae have been found. They occur singly on the ventral side of certain pores for the tube-feet, and take the form of a pedicle bearing two hook-like valves which are however not apposed but placed side by side. The arms are distinct at their base; they are branched in the Astrophytidae, and in this family as well as in other Euryalida their extremities, owing to the absence of calcareous shields, can be rolled up towards the mouth. They are covered in the Ophiurida by a single series of dorsal shields, of right and left lateral or adambulacral shields, and of ventral, super-ambidacral or ambtdacral shields. A pair of large shields, termed 'radials' but not to be confused with the apical radials, overlie the bases of the arms dorsally. The adambulacral shields are large, their distal margin projecting and generally provided with spines. An aperture between the edge of one shield and of its successor, and the corresponding super-ambulacral gives exit to the tube-feet. It is often surrounded by a ring of small plates. The lateral shields appear about the same time as the vertebral ossicle: and, in Amphiura, the ventral a little before the dorsal shield.

The terminal plate becomes tubular and grows round the tentacle (azygos tube-foot) at the tip of the arm. Within the arms is a linear series of large more or less disc-like vertebral, ambulacral, or sub-ambulacral ossicles, placed dorsally to the radial water-vascular trunk.

These ossicles take up the greater part of the cavity of each arm. Each one is composed of a right and left half immoveably united by suture. In certain deep-sea Ophiurida, however, each half retains the embryonic character of a curved bar united to its fellow at either end. The ossicles are connected by well-developed articulations, and there is a series of intervertebral, dorsal and ventral, muscles longitudinally disposed by means of which the arms are actively moved, principally in a horizontal plane. The halves of the two first pairs of ossicles are separated. The right half of the first ossicle in one arm approaches the left half of the corresponding ossicle in the adjoining arm to form a paired interradial peristomial plate which is internal, and placed dorsally to the triangular oral angle pieces. Each angle piece projects interradially, and the mouth has consequently a stellate figure. The piece consists of a right and left half united moveably by a joint. And each half has its apex formed by the first adambulacral which is borne upon a support, the second ambulacral of its own side. This second ambulacral is covered externally by the second adambulacral or lateral buccal shield which lies at the side of the buccal shield or oral plate.

The ventral margin of each angle piece supports spines, the buccal papillae at the sides, the dental papillae at the apex of the triangle. The vertical oral edges of the conjoined first adambulacrals support a row of spines, the palae angulares with basal pieces. The latter generally fuse to form a bar, the torus angtdaris. The second ambulacral is pierced by the two first tube-feet or buccal feet which protrude into the cavity of the mouth. An aperture limited by the first and second adambulacrals and the first superambulacral shield gives exit to the third tube-foot.

The nerve ring lies on the inner or oral surface of the peristomial plates. The five radial nerves immediately underlie the superambulacral shields. Branches pass outwards on the tube-feet in which there is a sub-ectodermic nerve layer; and such a layer may perhaps exist beneath the ectoderm of the body-surface. Two genital nerves arise distally to the branches for the buccal feet. The blood-vascular ring and radial vessels lie beneath the corresponding nerve structures. There is an aboral ring or plexus which alternately ascends dorsally on the arms, and descends ventrally in the interradii. The rings and vessels are inclosed between two perihaemal spaces; a perihaemal space also incloses the stone-canal, and the plexiform organ which connects the two rings in the madreporic interradius. The water-vascular ring has a ciliated stone-canal ending with an ampulla on one of the buccal shields which is pierced by a pore. Some species of Astrophyton have five madreporites (pore plates), one in each interradius. Trichaster elegans is remarkable for possessing five simple pores instead of pore-plates. The stone-canal in these cases is similarly multiplied. The ring sometimes possesses four Polian vesicles, one in each interradius except that which contains the madreporite.

The vesicles may be replaced by ramified caecal tubules (vasa ambulacralia cavi). The radial vessels lie in a groove on the ventral surface of the ambulacral ossicles. The branches to the tube-feet perforate the aboral end of the ambulacral ossicles. The feet are simple and conical, and there are no ampullae. The two first pair, or buccal feet, are derived from a branch of the ring itself which bifurcates. Delicate calcareous plates are found in the walls of the ring and feet. Globular cells tinged with haemoglobin occur in the water-vascular system of Ophiactis virens.

The mouth is hidden by the interradial oral structures. The digestive tract is globular with radial expansions, which do not, however, extend up the arms. It is lined by ciliated epithelium, and is fixed to the body walls by connective tissue threads which cross the coelome. This cavity is not large, and extends outwards above and below the ambulacral ossicles. The genital organs consist of an infertile rhachis contained within the aboral blood plexus, and connected to a numerous set of fertile genital caeca, which are disposed in a vertical series, and open on either side of a genital bursa close to its aperture. These bursae have delicate walls of invaginated perisoma, which sometimes contain calcareous plates. They extend inwards dorsally to the digestive sac, and open by slit-like ventral apertures, one on either side the base of an arm. There are therefore two bursae in each interradius. The bursal aperture is divided in- Ophioderma into an adoral and an aboral part; the bursa itself is single, but the genital caeca are confined to the dorsal portion, and to the interradial wall. Currents of water flow in and out of the apertures, and the bursae have probably a respiratory function. The adradial wall of each bursa is supported by a genital plate.

The genital caeca are surrounded by a blood plexus derived on the interradial side from the aboral ring, on the adradial from a special genital vessel.

The free swimming larva is a Pluteus, and differs from the Echinoid Phiteus in possessing a pair of lateral arms developed from the sides of the anal portion of the ciliated ring.

Some Ophiurida develope a phosphorescent light from the dorsal aspect of the arms. Both sub-groups of the Class appear in the Silurian strata.

There are two sub-groups

1. Euryalida: devoid of plates in the integument; with simple or branched arms, which can be rolled up towards the ventral aspect at their tips; pedicellariae present. Astrophyton, Trichastef, and Astronyx are the principal genera.

2. Ophiurida: with simple arms, superambulacral shields, and five buccal shields; the disc has either a soft integument or is covered by granules or plates. There are several families.

See lit., pp. 194, 196.

Formation of arm skeleton, Ludwig, Z. W. Z. xxxvi. 1882. Pedicellariae in Euryalida, Id. Z. W. Z. xxxi. 1878.

Apical system, P. H. Carpenter, Q. J. M. xxiv. 1884. General anatomy, Id. Q. J. M. xxi. 1881.