The animal consists of a central disc, which is prolonged into five lobes, the so-called arms, rays, or radii. The interval or part between each radius is known as an interradius. Two surfaces may be distinguished: - one flat or somewhat concave, the ventral, oral, actinal, or ambulacral surface; the other convex, and termed the dorsal, aboral, abactinal, or anti-ambulacral surface. In the centre of the ventral aspect of the disc is the membranous peristome with the mouth. Five sets of spines, the mouth-papillae, project over this area interradially, giving it a pentagonal appearance: and there radiate from it the five avenues or ambulacral grooves, one to each ray, which lodge the locomotor feet and hence give this aspect of the animal the name of ambulacral. The feet in question have been removed from two of the grooves for a short space, but are left in situ and in a dried condition elsewhere. Examining the exposed part of each groove attentively, it is seen to be formed by two series of narrow parallel ossicles - the so-called ambulacral or vertebral ossicles, the long axes of which are at right angles to the axis of the ray.
The ossicles of one side of the groove are inclined at an obtuse angle, open ventrally, to the corresponding ossicles of the other side; and their dorsal ends are articulated moveably together. The summit of the angle is median. The groove lodges the radial water-vascular vessel, the inferior transverse vertebral muscles, the radial perihaemal spaces and bloodvessels, and the radial nerve-cord, with the feet. Between the ossicles are a series of pores, one pore between each pair, formed by the apposition of two grooves in adjoining ossicles. The ampullae of the feet which lie on the dorsal side of the ossicles communicate through these pores with the ventrally placed feet. The two first pores lie in the same straight line, while the succeeding, to very near the tips of the arms, are arranged in a zig-zag fashion, being alternately near to, and remote from, the axis of the ray. Hence there appear to be four rows of pores and four rows of feet to correspond. In the majority of Asteroidea, however, the pores retain a straight linear arrangement for the whole extent of the grooves.
The edges of a groove are bordered immediately by a series of fine moveable spines, borne by the adambulacral ossicles (infra). In this specimen there are three rows of such spines, but in many instances there are only two. At the oral end of each groove they form the mouth-papillae above-mentioned, the spines of one side in one groove meeting the spines of the adjoining side of the contiguous groove interradially. Externally to this series of spines comes another series in triple row of stout spines, fixed like all the other spines of the body. They are borne by the median set of interambulacral ossicles (infra). This series of spines, and the series of moveable spines, both extend to the tip of the ray, ceasing at a spot where a circlet of spines denotes the position of the eye-speck and terminal feeler or tentacle in the living animal. A third series of stout spines in a single row, borne by the inferior marginal ossicles (infra), borders the ventral aspect of the ray on either side.
Turning to the dorsal or aboral surface the perisoma or integument with its network of calcareous ossicles and membranous soft intervals may be first noticed. The spine-bearing ossicles form more or less regular lines parallel to the axes of the rays. One line in the middle must be noted particularly. Attached to the soft intervals and at the bases or tips of the spines may be seen scattered pedicellariae. Other pedicellariae are grouped round and on the series of moveable spines of the ventral surface. These two sets of pedicellariae differ remarkably from one another. Both however are to be regarded as modified spines, not zooids, like the polymorphic aviculariae and vibracula of Polyzoa. (See Flustra, Preparation 48, post.) In one of the interradii is a circular calcareous plate, the madreporic tubercle or madreporite, the surface of which is marked by grooves radiating from the centre. By the removal of the perisoma from the disc, it may be seen that a canal with calcareous walls - the stone-canal, - curved like the letter S, leads from this plate to the ventral surface, where it opens into the circum-oral water vascular vessel. The two rays, one on either side the madreporic plate, constitute the bivium as ordinarily defined; the three remaining rays the trivium.
The central one of the three lies opposite the madreporic interradius and is often spoken of as the anterior ray.
The perisoma has been removed from the dorsal surface of three rays, and the ambulacral ossicles can be seen from their dorsal aspect. Their median ends form a prominent vertebral ridge with median furrow.
Each first ambulacral ossicle is large, broad, and pointed medianly, and projects over the peristome radially. It appears to be formed by the fusion of two ossicles. The pore is large. At the outer end of this ossicle and to the outer side of the pore is an enlarged first adambulacral ossicle, which is, however, smaller than the ambulacral. This adambulacral with its fellow adjoining the same interradius carries the oral papillae or spines which project over the peristome interradially. In the majority of Starfish the first adambulacral itself projects interradially over the peristome beyond the corresponding ambulacral ossicle and with its fellow in the adjoining ray forms two teeth. The ambulacral is in this case a support for the teeth. The first type of mouth is termed ambulacral, the second adambulacral. The former appears to be characteristic of those Starfish in which the ambulacral pores are arranged in zig-zag; the latter of those in which they maintain a linear arrangement. A small plate, somewhat indistinct in this specimen, overlies interradially each set of first adambulacrals.