The main peculiarities in the structure of Echinarachnius are comprehensible when the species is viewed from above in its normal environment. It is found in comparatively shallow water on a sandy bottom, and normally is nearly but not quite buried in the sand. It might thus be overturned by the force of the waves and currents, and it is protected against this fate by its flattened shape. This shape, however, necessitates some kind of support for the upper part of the test, and this is provided by the internal partitions.
In order to view the internal anatomy of the "Sand-dollar," it is necessary carefully to pick away the dorsal surface of the shell piece by piece. In this way the whole course of the alimentary canal is exposed; as in Echinus esculentus it can be seen to issue from the upper surface of Aristotle's lantern. It then bends sharply to the left, and makes a complete circle round the edge of the disc; this portion is the stomach, and is considerably inflated and accompanied by a "siphon." It then bends sharply back on itself, but only goes half way round; when it reaches the posterior interradius it ends in the anus (Fig. 244).
Aristotle's lantern is greatly simplified as compared with its condition in the Regular Urchins. Both rotulae and compasses are absent; the jaws are sharply bent on themselves, and their appearance gives one the impression that they have shared in the process of compression which the test as a whole has undergone, and have thus become bent. The teeth are nearly horizontal, and they actually articulate with the auriculae, which, as in Cidaridae, consist of disconnected pillars and spring from the plates of the interradius. Each pillar is fused with the adjacent one belonging to the next radius, so that the system which in Echinus consists of five radial arches here consists of five interradial pillars. Aristotle's lantern has lost its respiratory function and apparently its masticatory function as well, for the teeth are used as spades to shovel into the mouth the sand mixed with organic detritus and small organisms on which the animal lives.
The water-vascular system is highly modified. There are two sharply marked kinds of tube-feet—(a) the respiratory tube-feet, (b) the locomotor tube-feet. Both kinds are terminated by suckers, but the first variety are much larger than the second; they possess a flattened lobed base, and are connected with the ampulla by a double canal. They issue only from the double pores which form the petal. The locomotor tube-feet are small and cylindrical; they are, as already mentioned, scattered over the whole upper surface of the test, penetrating both ambulacral and interambulacral plates, but all are connected by transverse canals with the radial canals of the water-vascular system. On the under surface they are confined to the neighbourhood of the ambulacral grooves, which have nothing to do with the ambulacral grooves of an Asteroid, but are due to secondary localisations of the tube-feet, which are here also connected in each radius with a single radial canal. The appearance of a living Echinarachnius covered with a veritable forest of short brown tube-feet is very striking.[[500]]
Fig. 244.—Dissection of Echinarachnius parma. × 1. The oesophagus has been cut through and moved to one side so as to expose Aristotle's lantern. The aboral part of the test has been removed. gon, Genital organ; int, intestine; musc, transverse muscle connecting jaws of adjacent interradii; rect, rectum; siph, siphon; st, stomach.
The condition of the water-vascular system is to be explained entirely by the peculiar environment of the animal. The demand for specialised respiratory organs is brought about by the habit of living half buried in the sand. Under these circumstances the strain of supplying the needful oxygen is thrown on the dorsal tube-feet, and they become modified in order to fit them for this function. The locomotor tube-feet are very small and feeble compared with those of Echinus esculentus, but this is comprehensible when it is recollected how little resistance the yielding sand would offer to the pull of a powerful tube-foot like that of the Regular Urchins, for in order to move the creature through the sand a multitude of feeble pulls distributed all over its surface is necessary, and the locomotor tube-feet are exactly fitted, both as to size and number, for this object.
The principal points in which Clypeastroidea vary amongst themselves are (1) the nature of the internal skeleton, (2) the shape, and (3) the spines.
Internal Skeleton.—In Echinocyamus and its allies this consists in each interradius of two simple partitions radiating out towards the edge of the disc; in Laganum it consists of walls parallel to the edge of the disc; in Clypeaster, of isolated pillars.
Shape.—In Echinocyamus the outline is oval and the test comparatively high. In Clypeaster and its allies the outline is pentagonal, and the test is swollen up into a blunt elevation in the centre. In a large number of genera, however, the test is, as in Echinarachnius, extremely thin and flat, and the outline may be variously indented. A first indication of this process is seen in Echinarachnius itself, but in Rotula the edge is drawn out into finger-like processes which are all interradial. In Mellita these processes unite with one another distally so as to surround spaces called "lunules," which appear as perforations of the test.