Vermiform or slug - like Echino-derms, with a leathery skin, in which calcareous granules and spicules are generally developed. Mouth surrounded by a circlet of tentacles. Sexes mostly distinct. Larva vermiform, without a skeleton.
The members of this order are commonly known by the name of "sea-cucumbers," "trepangs," or "beches-de-mer," and are the most highly organised of all the Echinodermata. The body is elongated and vermiform, or rarely slug-shaped, and is not provided with a distinct test, but is enclosed in a coriaceous skin, generally, but not always, containing calcareous deposits in the form of scattered granules or spicules, or even imbricated scales. The ambulacral tube - feet, when present, are typically disposed in five rows, which divide the body into an equal number of longitudinal segments or lobes. The mouth is surrounded by a circlet of feathery tentacles, containing prolongations from the central ring of the water-vascular system; and an anus is situated at the opposite extremity of the body. There is a long, convoluted intestine. A special respiratory, or water-vascular, system is often developed, in the form of a system of arborescent tubes, which admit water from the exterior. The larva is vermiform, and has no skeleton. At a certain period of their existence, the young Holothurians are barrel-shaped, with transverse rings of cilia (fig. 109, c).
Fig. 109. - Holothuroidea. a Holothuria tubulosa, one of the Sea-cucumbers b and c Young stages of the same.
They rotate rapidly on their long axis, and have at this stage been described as a distinct genus under the name of Auri-cularia.
In the typical Holothurians, locomotion is chiefly effected by means of rows of ambulacral tube-feet, or by alternate extension and contraction of the worm-like body; but in the Synaptidae, in which there are no ambulacra, and only the central circular canal of the ambulacral system is present, the animal moves by means of variously shaped spicula, which are scattered in the integument. When developed, the ambulacral system consists of a "circular canal,' surrounding the mouth, bearing one or more "Polian vesicles," and giving off branches to the tentacula; and of five "radiating canals" which run down the interspaces between the great longitudinal muscles. These radiating canals give off the tube-feet and their secondary vesicles, just as in the Echinus. In the typical forms there are five rows of tube-feet, but these organs may be scattered over the whole body, or may be restricted to the ventral surface. There is also a "sand-canal" which arises from the circular canal, and is terminated by a madreporiform tubercle; but this, instead of opening on the exterior, hangs down freely in the perivisceral cavity. The fluid, therefore, with which the ambulacral system is filled, is derived from the perivisceral cavity, and not from the exterior, as is usually the case.
The mouth is toothless, is situated anteriorly, and is surrounded by a beautiful fringe of branched, retractile tentacles (fig. no), which arise from a ring of calcareous plates, and into which are sent prolongations from the circumoral ring of the ambulacral system. These tentacles, ten to twenty in number, are really modified tube-feet, and probably serve in part as respiratory organs. The mouth opens into a pharynx, which conducts to a stomach. The intestine is long and convoluted, and usually opens into a terminal dilatation, termed the "cloaca," which serves both as an anus and as an aperture for the admission of sea-water to the respiratory tubes. From the cloaca arise, in many forms, two branched and arborescent tubes, the terminations of which are caecal. These run up towards the anterior extremity of the body, and together constitute the so called " respiratory tree." They are highly contractile, and they perform the function of respiratory organs, sea-water being admitted to them from the cloaca. The vascular system consists of two main vessels - one dorsal, and the other ventral - connected with a circum-oesopha-geal ring. Development is in a few instances direct; but in most cases there is a metamorphosis, the larva being vermiform, and devoid of a skeleton. The nervous system consists of a cord, surrounding the gullet, and giving off five branches which run alongside of the radiating ambulacral canals. The sexes are generally, but not universally, distinct. The generative organs are in the form of long, ramified, caecal tubes, which open externally by a common aperture, situated near the mouth. There is thus no trace of that radial symmetry which is observed in the arrangement of the reproductive organs in the other orders of the Echinodermata.
Fig. 110. - Pentacta frondosa, showing the crown of feathery tentacles round the mouth and the rows of tube-feet.
The skin in the Holothurians is highly contractile, and the body is provided with powerful longitudinal and circular muscles, in compensation for the absence of any rigid integumentary skeleton. Many of the Sea-cucumbers, in fact, are endowed with such high contractility that they can eject their internal organs entirely, if injured or alarmed.
In the family of the Synaptidae there is no respiratory tree, and the ambulacral tube-feet are wanting; whilst the skin is furnished with calcareous spicules of various shapes. The Synaptae themselves burrow in the mud or sand, and have the skin furnished with innumerable anchor-shaped spicules (fig. 111) attached to special "anchor-plates" in the integument. They often form a kind of protective case or tube of sand-grains; and they obtain their food by swallowing the mud, from which they extract any disseminated nutrient particles. In Chirodota the skin is provided with microscopic calcareous wheels, in the place of anchors. In the Oncinolabidae) the skin has barbed spicules, and there is no respiratory tree; but these forms differ from the Synaptidae proper in possessing tube-feet.
The order Holothuroidea is divided into the two following sub-orders:
Sub-order I. Apneumona.
No respiratory tree. The ambulacral tube-feet wanting (Synaptidae), or present (Oncinolabidae). Sub-order 2. Pneumonophora.
A respiratory tree. (Ex. Holothuria, Thyone, Molpadia, Psolus, Cu-cumaria, etc.)
Fig. 111. - Anchor-shaped spicules of Synapta, and the plates to which these are attached. Magnified greatly.