Body stellate, consisting of a central "disc," in which the viscera are contained, and of elongated "arms" which are sharply separated from the disc, solid, not containing prolongations of the viscera, and not furnished inferi-orly with ambulacral grooves. Larva generally pluteiform, with a skeleton.
This order comprises the small but familiar group of the "Brittle-stars" and Sand-stars," often considered as belonging to the Asteroidea, to which they are nearly allied. The body in the Ophiuroidea (fig. 100) is discoidal, and is covered with granules, spines, or scales, but pedicellariae are wanting. From the body - which contains all the viscera - proceed long slender arms, which may be simple or branched, but which do not contain any prolongations from the stomach, nor have their under surface excavated into ambulacral grooves. The arms, in fact, are not simple prolongations of the body, as in the Asteroidea, but are special appendages, superadded for locomotive and prehensile purposes. Each arm is enclosed by four rows of calcareous plates, one on the dorsal surface, one on the ventral surface, and two lateral. The lateral plates generally carry more or less well-developed spines. In the centre of each arm is a chain of quadrate ossicles, forming a central axis, and between this axis and the row of ventral plates is placed the ambulacral vessel. Each ossicle of the central chain is composed of two symmetrical halves, but these are immovably articulated together, and are not movable upon one another, as in the Asteroidea. The mouth is situated in the centre of the inferior surface of the body, is provided with a masticatory apparatus, and is surrounded by tentacles. It opens directly into a sac-like ciliated stomach, which is not continued into an intestine, the mouth serving as an anal aperture. The stomach is destitute of lateral diverticula. The reproductive organs are situated near the bases of the arms, and open by orifices on the ventral surface of the body or in the interbrachial areas.*
The ambulacral system is constructed upon the same plan as in the Echinoids and Asteroids; but its place as a locomotive apparatus is taken by the arms. The radial vessels of the ambulacral system are not provided with secondary vesicles or "ampullae," as they are in the Echinoidea and Asteroidea, and the lateral "feet" which they give off have no terminal suckers. The madreporiform tubercle is placed on the inferior surface of the body, and is often partially concealed by one of the plates surrounding the mouth.
Respiration is carried on by the lining of the body-cavity, and by a circlet of modified tube-feet or tentacles placed round the mouth.
The development of the Ophiuroids is sometimes direct, the young being brought forth alive, and, in some cases, being carried by the mother for some period after hatching (Wyville Thomson). More commonly there is a pluteiform embryo, which resembles that of the Echinoids in having a continuous endoskeleton.
In Euryale the body is in the form of a subglobose disc with five obtuse angles, and the arms are prehensile. In Asterophyton, the Medusa-head star, the arms are divided from the base, first dichotomously, and then into many branches. In Ophiura, the Sand-star, the arms serve for reptation (creeping), and are undivided, often exceeding the diameter of the disc many times in length.
* Spontaneous fission has been observed by Lutken and Kowalewsky to take place in some Ophiuroids, as also occasionally in some of the Asteroids.
Fig, 100. - Ophiuroidea. a Ophiura texturata, the common Sand-star; b Ophiocoma neglecta, the grey Brittle-star. (After Forbes.)
The order Ophiuroidea may be divided into two families, as follows:
Family I. Euryalidae.
Arms branched ; genital fissures ten in number. Fam. 2. Ophiuridae.
Arms simpl ; genital fissures, mostly five in number.