Order I. Aspidochirota.
Holothuroidea with shield-shaped feelers provided with ampullae; with radial canals and numerous podia and with respiratory trees. Retractor muscles absent. Nearly a third (158) of the species of Holothuroidea belong to this order, but there are only six genera, and of these Holothuria includes no less than 109 species. The Aspidochirota seem for the most part to live on somewhat firm ground, the surface of which they are continually sweeping with their shield-shaped feelers, which brush the adherent organisms into the capacious mouth. Four species of Holothuria—viz. H. intestinalis, H. tremula, H. aspera, and H. nigra are recorded from British waters. The first-named is a northern form, distinguished by the fact that all its podia have suckers; it is found in the north of Scotland. H. tremula is intermediate in structure between H. intestinalis and H. nigra, and is found in deep water off our western coasts. H. aspera, remarkable for the radiating spines growing out from its ossicles, has been recorded only once from deep water. Of the other genera it is only necessary to mention Stichopus, remarkable for the square outline of its transverse section, and for the restriction of the ventral tube-feet to the radii; there is also a well-marked tapering of the anterior end, so that this genus may be said to have a neck. Stichopus is almost entirely confined to tropical waters, and some of its species, as also species of the ubiquitous genus Holothuria, as well as many other undetermined species, constitute the valuable "Trepang," which is a delicacy much prized by the Chinese. The Trepang are caught in various parts of the Malay Archipelago. They are cooked in sea water to preserve them, dried in the sun, and boiled in fresh water repeatedly, till all the salt is extracted. They are then dried and sent to market, where they are used in making soup.
Order II. Elasipoda.
Holothuroidea with shield-shaped feelers, destitute of retractor muscles; all the podia have more or less pointed ends,[[509]] but there is a marked contrast between dorsal and ventral podia, and the ventral surface is flattened so as to constitute a creeping sole. No respiratory trees, at most a simple diverticulum of the intestine; frequently the primitive external madreporite is retained, and contains several pores.
A number of spherical sacs containing little spherical calcifications (otocysts) are attached to the nerve-ring in some genera. Can these be metamorphosed sphaeridia of Echinoid ancestors?
Fig. 258.—Ilyodaemon maculatus. × ⅔. (After Théel.)
The first member of this remarkable order to be discovered was Elpidia, which was dredged in 1875 by the Swedish Arctic Expedition, and described by Théel.[[510]] The majority of the known members of the order were discovered by the dredging expedition of H.M.S. "Challenger." The species composing it are, with one exception, inhabitants of what may be termed the abysmal depths of the sea. The exception alluded to (Ilyodaemon maculatus) is confined to the belt between 100 and 150 fathoms in depth. The well-marked sole and the absence of suckers point to a life consisting of constant peregrinations over the soft ooze forming the ocean floor. The ooze forms their food, and as their weight must to a certain extent immerse them in it, we can understand why the stiff, long dorsal podia have been specialised as respiratory organs, since there are no respiratory trees. These respiratory podia sometimes undergo extraordinary development; thus in Peniagone several very long ones cohere to form a huge vertical sail, whilst in Psychropotes one or two cohere to form a backwardly projecting tail. On the other hand, in Ilyodaemon (Fig. 258) the dorsal podia are numerous and slender.
Order III. Pelagothuriida.
Holothuroidea with shield-shaped feelers provided with long ampullae which project outwards, pushing the skin before them so as to form external appendages, connected at the base by a web. Calcifications absent. No retractor muscles. No respiratory trees. The external madreporite is retained, but all podia other than the feelers have disappeared, although the radial canals have been retained.