Fig. 241.—Oral view of "Sand-dollar" (Echinarachnius parma), with spines. amb, Ambulacral furrow, × 1.
The whole test is covered with extremely short delicate spines, which form a velvety felt-work, and are all of approximately the same length; they are of a brownish purple colour. The spines on the dorsal surface are all ciliated, and these cilia cause a current of fresh sea-water to flow continually over the modified tube-feet. Pedicellariae are scattered amongst the bases of the spines; they are of the tridactyle, the gemmiform, and the ophicephalous types, but they have only two jaws.
Fig. 242.—Aboral view of the "Sand-dollar" (Echinarachnius parma), with its spines. m.p, Madreporite; pod, small tube-foot with sucker; pod', flattened respiratory tube-foot. × 1.
The ambulacral areas on the upper surface of the test can be distinguished only by the flattened respiratory tube-feet (Fig. 242, pod'), which can be seen protruding from between the spines. Below these areas are clearly marked, for in the centre of each is a well-marked groove proceeding inwards to the peristome. This groove receives lateral branches on its course which traverse the adjacent interambulacral regions. The purpose of these grooves will be explained later. The interambulacral regions do not reach the peristome, which is entirely surrounded by the ambulacral areas. The ambulacral and interambulacral areas both consist of somewhat large hexagonal plates, except in the region of the respiratory tube-feet. Here the pore-plates are not united with one another. This region in each radius is termed a "petal" (Fig. 243, A, p), for the respiratory tube-feet are arranged in two rows which diverge from their commencement at the "calyx" and slightly converge again towards the outer margin of the disc, and thus in a dried specimen the two rows of double pores outline an area having some resemblance in shape to the petal of a flower. Besides these double pores for the larger tube-feet there are numerous small single pores for the smaller tube-feet; these are found in all the plates, ambulacral and interambulacral, of the dorsal surface, but in the neighbourhood of the grooves only on the ventral side.
Fig. 243.—A, aboral, B, oral view of Echinarachnius parma after spines have been removed, amb (in A), Ambulacral plates, (in B), ambulacral furrows; g.p, genital pore; inter, interambulacral plate; p, petal; t.t, pore for terminal tentacle, × ½.
The sphaeridia are only present to the number of one in each radius. Each sphaeridium is enclosed in a pit situated near the edge of the peristome.
A remarkable feature in the skeleton of Echinarachnius which is characteristic in greater or lesser degree of all Clypeastroidea is the presence of vertical partitions of calcareous matter traversing the coelom and stretching from the upper to the lower surface of the test. These are found principally in the peripheral region of the animal; and there can be no doubt that they have originated as cellular bands traversing the coelom, for the formation of similar structures can be followed step by step in the Crinoidea. In the axis of these trabeculae, or folds of the coelomic wall, jelly is secreted, and into this the lime-producing amoebocytes wander. In Echinarachnius these partitions are arranged in groups, each group radiating from a common centre.