Fig. 210.—Proximal and distal views of the three types of vertebra found amongst Ophiuroidea. A, Ophioteresis, a type of the Streptophiurae (after Bell), × 24; B, Astroschema, a type of the Cladophiurae (after Lyman), × 10; C, Ophiarachna, a type of the Zygophiurae (after Ludwig), × 3. The upper figure in all cases represents the distal aspect, the lower the proximal aspect of the vertebra. v.g, Ventral groove.
When the surface of a vertebra is examined it is found that it can be divided into a thin border, to which are attached the four muscles by which it is connected to its successor and predecessor, and a central portion, on which are situated the knobs and pits, by means of which it articulates with the next vertebra.
The simultaneous contraction of the two upper muscles causes the arm to bend upwards. The contraction of the two lower bend it downwards, whilst a sideward movement is effected by the contraction of the upper and lower muscle of the same side. On the proximal surface of the central portion of the vertebra there is a central knob and two ventro-lateral knobs, a median ventral pit and two dorso-lateral pits, and on the distal surface there are pits corresponding to the knobs on the proximal side and vice versa (Fig. 210, C). These knobs and pits restrict the movement of one vertebra on the next, so that although the arms can undergo an unlimited amount of flexion from side to side, they cannot be rolled up in the vertical plane. When the under surface of the vertebra is examined there is seen on each side of the central groove two round holes, a distal and a proximal. The distal pair are for the passage of the canals connecting the radial water-vessel with the tentacles, these canals traversing the substance of the vertebra for a part of their course; the proximal pair are for nerves going to the longitudinal muscles, which likewise perforate part of the ventral border of the vertebra.
In order to understand the anomalous circumstance that the canals going to the tentacles actually perforate the vertebrae, it must be clearly borne in mind that the basis of the body-wall in all Echinoderms is a mass of jelly with amoebocytes in it, to which we must assign the power of secreting carbonate of lime, and all we have to assume in the case of Ophiuroids is that calcification spread outwards from the original ambulacral ossicles into the surrounding jelly, enclosing any organs that happened to traverse it.
When the ossicles of the arm are followed inwards towards the mouth, they are seen to undergo a profound modification, so as to form, by union with the corresponding ossicles of adjacent arms, a structure called the mouth-frame. The general character of this modification is similar to that affecting the first ambulacral and adambulacral ossicles in the arms of an Asteroid, but in the Ophiuroid the change is much more profound. The first apparent vertebra consists of two separated halves, and each is fused with the first adambulacral (lateral) plate, which in turn is firmly united with the corresponding plate in the adjoining arm. Thus is formed the "jaw," as the projection is called. The extensions of the mouth-cavity between adjacent jaws are termed "mouth-angles." To the apex of each jaw is attached a plate bearing a vertical row of seven short blunt spines called "teeth" (Fig. 212, p). The plate is called the "torus angularis" (Fig. 211, T), and on its ventral edge there is a tuft of spines which are termed "tooth-papillae" (Fig. 208, t.p). On the upper aspect of the jaw are a pair of plates termed "peristomial plates." These discs—of which there are two in each radius, one on each jaw which flanks the radius—possibly represent the separated halves of the first vertebra, the apparent first vertebra being really the second. On the flank of the jaw there is dorsally a groove for the water-vascular ring and nerve-ring (Fig. 212, n.r), and beneath this a groove for the first tentacle and a pore for the second, both of which spring directly from the ring-canal; below these, in most Ophiuroidea, but not in Ophiothrix, there is a row of blunt triangular spines called "mouth-papillae" (Fig. 212, p1).
Fig. 211.—Diagrams to show the modification of the ambulacral and adambulacral ossicles to form the armature of the mouth. A, Asteroid; B, Ophiuroid. A1-A4, the first four ambulacra ossicles; Ad1-Ad4, the first four adambulacral ossicles; J1, the first plate of the interradius (in the Ophiuroid the scutum buccale); P, the spines borne by the jaw (in the Ophiuroid the teeth); T, the torus angularis; W, the water-vascular ring; Wr, the radial water-vessel; I, II, the first two pairs of tube-feet. (After Ludwig.)
The words "jaw" and "tooth" are misleading. There is no evidence that the jaws of a Brittle Star are ever used for crushing food, but by means of the muscles attaching them to the first complete vertebra in the arm they can be rotated downwards so as greatly to enlarge the mouth, and again rotated upwards and inwards, when they form an excellent strainer to prevent the entrance of coarse particles. To permit this extensive movement the articulatory facets on the proximal surface of the first vertebra have been much modified; the median knob and pit have disappeared, and the dorso-lateral pits are raised on to the surface of processes, so that there are in all four processes, two of which articulate with one half of a jaw.
