Holothurians

Holothurians, or Sea-cucumbers, one of the classes of the Echinodermata. They are elongated and worm-like in form, have soft bodies which may be strengthened by calcareous plates or spicules; they have a circle of tentacles around the mouth, and the madreporite, or opening of the water vascular system, hangs loosely in the interior. They are all marine, and live mainly among coral reefs, though one order (Elasipoda) inhabit the deep seas. As with most of the Echinodermata, the organs are arranged on a five-rayed plan. Thus there are five rows of tube feet, arising from five water vascular yessels. These are the usual means of locomotion, but in the Synaptidcc these are absent, and the Holothurian moves in a worm-like manner. Respiration is usually effected by n, circle of tube feet round the moiith, which have been modified to form a number of branched feathery tentacles. At the other end of the alimentary canal is usually one or a pair of "respiratory trees." There is no certain representative of the circles of five plates which form the apical system (q.v.) of other Echinoderms. Owing to this, and the primitive nature of some other characters, many authors have sought in this class for the ancestor of the Echinoderms. The class is divided into three orders: - 1. Elasipoda: Primitive deep-sea forms, which are shaped like a slug; no respiratory trees.

2. Pedata: With tube feet.

(a) Aspidoohirotoe: Tentacles with ampullec; e.g. Holothuria.

(b) Dcndrochirota?: Tentacles arborescent; e.g. Cucumaria.

3. Apoda: No tube feet.

(n) Pneuinonophorn: Respiratory tree loosely attached.

(b) Apneumona: No water vascular vessels or respiratory trees. As is natural with a group having some slight skeletal structures, they are of little interest geologically. Spicules are, however, known from the Carboniferous period upward, and are not uncommon in some formations.