Echinodermata with elongated bodies, and a tough integument with well developed muscular coats, and as a rule a feebly developed calcareous skeleton. There is a circle of circumoral tentacles borne by the water-vascular ring, and the madreporic tubercle is usually placed internally. The mouth is ventral or terminal; the anus ventral, terminal, or dorsal.
The body is often more or less vermiform, or in cross section pentagonal. In other instances one surface corresponding to three ambulacral vessels, or the trivium, and to the larval ventral surface, is distinctly flattened as in many Aspidochirotae; and the flattening may be carried so far as to form a creeping sole, as in Psolus and Colochirns among Dendrochi-rotae and all Elasipoda. The surface corresponding to the remaining two ambulacral vessels or the bivium - the larval dorsum - is then more or less convex. In Rhopalodina elegans the dorsal surface is extremely short so that the mouth and anus are closely approximated, while the ventral surface is large and convex. The body walls are in many Elasipoda produced into a distinct rim bordering the ventral surface, and in the family Psychropotidae the dorsum is produced, as in the larva, anteriorly to the mouth, which thus lies at some distance from the anterior extremity. The body walls consist of a cuticle, an ectoderm, a connective tissue layer of some thickness imbedding pigment and other cells, calcareous deposits and bundles of nerve fibrils; of a layer of circular or transverse muscles, and five radial bands of longitudinal muscles placed internally to the radial nerves and water-vascular vessels.
The circular muscles are sometimes interrupted in the radial lines, in Elasipoda, and except in this group, the longitudinal muscles consist in each radius of a right and left band. These bands are attached anteriorly to the radially placed pharyngeal ossicles, and they each give off in Dendrochirotae retractor muscles for the pharynx. The apical system of plates is not represented. The five calcareous plates in Miilleria, etc, or the ten in Rhopalodina which surround the anus have probably nothing to do with it. The oral system is represented by five calcareous valves which close over the mouth and tentacles in Psolus ephippiger, or the young Psolinus brevis. It is perhaps present also in some larval forms. The calcareous deposits of the body are as a rule represented only by scattered spicules as in most Elasipoda, by wheels ( = rotulae), e.g. in Chirodota, by anchors with plates in Synapta or variously shaped plates, all however small in size. Well developed calcareous plates are found in some instances, e. g. in Ocnus and Psoitis; and in Echinocucumis and Echinosoma they carry spines.
Overlapping plates have been observed in a larval Holothuria and Cucumaria. Spicules are commonly found in the walls of the tube feet, and in Elasipoda they occur in various tissues, e. g. mesentery, walls of digestive tract, etc. The body walls are lined by a ciliated epithelium which is continued over the mesentery. The coelome is large, and a special section of it, the oesophageal sinus, surrounds the pharynx. The pharyngeal ossicles are formed in the outer wall of this sinus. They are sometimes absent (Embolus) or very rudimentary. Among the Elasipoda, the Elpidiidae have five radially placed spicules; the Psychropotidae five plates (?), and the Deimatidae a ring-shaped network. Typically there are five radially placed ossicles, notched or perforated, by the radial nerves and water-vascular vessels, alternating each with a single interradial ossicle, but the number of the latter varies and may be increased with an increase in the number of the tentacles in Apneumona. The pharynx of some Dendrochirotae is protected by a row of calcareous plates.
The circumoral nerve-ring not only gives off the radial nerves but other nerves to the tentacles. There appears to be a plexus with ganglion cells in the integument, and the nerves to the tube-feet and tentacles are in some instances connected with sense-cells in the ectoderm. The dorsal processes of Elasipoda are especially rich in nerves, and are probably tactile in function. Auditory vesicles, ten in number, have been found at the origin of the radial nerves from the ring in Synapta. Similar structures occur in the Elpidiidae, and are either confined to the ring, or spread also along the two lateral nerves of the trivium. They inclose numerous otoliths. The five caeca first developed by the water-vascular ring in the larva appear to correspond with the five primary tentacles, and are inter-radial. A second set of five caeca develope into the radial water-vascular vessels. The tentacles increase in number subsequently, and in the adult vary from ten to thirty, but their interradial origin is not evident, and the ring appears to give off five vessels from which the tentacular branches and radial vessels are alike derived. In Thyonidium the ten tentacles are alternately large and small, and in the Dendrochirota the ventral pair are constantly of a smaller size than the remainder.
They vary much in shape. In Haplodactyla they are cylindrical, but they may be peltate (Aspido-chirotae), arborescent (Dendrochirotae), and peltate, digitate or pinnate in others. Tentacular suckers are found in some Synaptidae. The tentacles are sometimes retractile, and this is especially the case in Dendrochirotae; in which the pharynx has special retractors. There are ampullae to the tentacles in Aspidochirotae, Molpadia, Chirodota, etc., but never in Elasipoda or Dendrochirotae. There is generally one Polian vesicle which is ventral, and on the left side in Elasipoda, but in some forms the number may be greatly increased. The radial water-vascular trunks are absent in Apneumona; present but devoid of feet in Apoda Pneumonophora; and the same is the case with the two vessels of the bivium in Psolus. The tube feet are either partially or completely retractile, and furnished with a terminal disc which is supported by a calcareous plate except in the majority of Elasipoda, or they are conical papillae without discs. Both kinds may occur in the same animal, e. g. Stichopus, or the first kind alone, e. g.