Cucumaria, or the second alone, e. g. many Holothuriae. Among the Elasipoda the median ambulacrum of the trivium has rarely any feet, and the lateral ambulacra carry cylindrical feet, as a rule few and paired from side to side of the ventral surface. The papillae or conical processes are usually dorsal. They are well developed in most Elasipoda, and are paired from side to side. In the same group there is often a long dorsal appendage with a broad base, which extends across the median dorsal interambulacrum anteriorly, in the middle, or posteriorly, and is penetrated by processes from the two radial vessels of the bivium. The tube-feet may be arranged in rows or scattered over the surface irregularly as in Dendro-chirotae Sporadipoda, and they are frequently less numerous in the bivium. They possess ampullae which project slightly from between the circular muscles. In Elasipoda there are spaces branched or unbranched in the integument connected with the feet and dorsal papillae ( = processes, Theel), and true ampullae are rarely found and only in connection with the dorsal processes. The stone-canal is generally single but may be multiple or branched.
In many Elasipoda it opens on the dorsal surface medianly by usually one or rarely several pores close to the generative pore; and when it is retracted into the coelome, it does not hang freely but is partly imbedded in the integument. It hangs freely in the coelome or is attached to the mesentery in other Holothurioidea. It may or may not be terminated by a calcareous madreporic plate, but whether it is or not, it is covered by coelomic epithelium. Numerous corpuscles occur in the water-vascular system. The blood-vascular system consists of a series of spaces in the connective tissue, not lined by an epithelium, and disposed as a circular peripharyngeal plexus closely connected to the water-vascular ring, but placed posteriorly to it, and a ventral and dorsal intestinal vessel, the latter usually double except in Elasipoda. They are united by anastomoses round the intestine. The ventral vessel has been observed to contract from the centre to either end; the dorsal is connected to a plexus in the mesentery by which the left respiratory tree is loosely invested in Aspidochirotae and Apoda Pneumonophora.
The mouth is placed centrally in a peristome surrounded by the circle of tentacles. It is very frequently turned to the ventral surface; always in Elasipoda, and to an especial degree in the family Psychropotidae. It is in many instances terminal or even turned dorsally, and opens into a pharynx. The digestive tract is straight in some Apneumona, but is generally disposed in a descending, an ascending and a descending section. The first section is supported by a mesentery which is attached to the median dorsal line interradially, and corresponds to the dorsal line of union between the right and left peritoneal vesicles of the larva, the ventral union being absorbed. The ascending section is borne by a mesentery attached to the left dorsal interradius, and the second descending by one attached to the right ventral interradius. The tract is therefore slightly coiled. The pharynx is followed by the short 'stomach/ which has well-developed muscular walls, and is separated by a constriction from the intestine by far the longest part of the tract. The last portion or rectum is dilated, forming the so-called cloaca, which has been observed to contract rhythmically. Its walls are connected to the integument by radial muscles.
There are appended to it, except in Apneumonaand Elasipoda, two (rarely more) branched outgrowths - the respiratory trees - homologous with the two interradial intestinal caeca of Asteroidea. The mesentery in the Apneumona bears, especially at its base, a number of ciliated funnel-shaped bodies either scattered or in groups. Processes known as Cuvierian organs are attached to the cloaca. They are invariably absent in Synap-tidae, and perhaps also in some others. They are discharged when the animal is irritated, and have been variously stated to be solid or to contain an axial cavity.
The generative organs consist of either a single bundle of caeca attached to the left side of the mesentery anteriorly, or of two bundles right and left (Stichopus among Aspidochirotae, Dendrochirotae, and many Elasipoda). The duct is single and opens by a single pore placed in the dorsal median line outside the circle of tentacles or within it in Dendrochi-rotae. In some Elasipoda, however, it branches and opens by a corresponding number of pores. The sexes are separate with the exception of the Apoda Apneumona and Pneumonophora (?).
Psolinns developes without a metamorphosis, and the embyro is not ciliated. Cucumaria passes at once into the pupa stage, while others, so far as observed, pass through an Auricularia into a pupa stage; see p. 548. The larval mouth and anus persist in the adult.
Holothurioidea are found in all seas. The Aspidochirotae swallow quantities of sands, etc.; others feed on small animals. Fossil wheels (of a Chirodota)) have been found in Jurassic strata, and remains of a Chiro-dota and Synapta even as early as the Carboniferous strata in Scotland.
The Holothurioidea may be divided into 1. Elasipoda: primitive deep sea forms, with a well-marked bilateral symmetry, tube feet restricted to the flat ventral surface, and papillae on the dorsum. The stone-canal often opens externally by a pore, and there are no respiratory trees. (Elpidiidae, Deimatidae, Psychroprotidae).
2. Pedata, with well-developed tube feet and papillae, subdivided into
(a) Aspidochirotae, with peltate tentacles provided with ampullae, with five radial and five interradial pharyngeal ossicles, and left respiratory tree loosely connected to the mesentery; e. g. Holothuria, Stichopus.
(b) Dendrochirotae: with arborescent tentacles, retractor muscles to the pharynx, and two sets of generative caeca; e. g. Cucumaria, Psolus.
3. Apoda, devoid of tube feet and papillae, subdivided into
(a) Pneumonophora (= Molpadidae), with the left respiratory tree connected as in 2. a; ? hermaphrodite.
(0) Apneumona (= Synaptidae): devoid of radial water-vascular vessels, respiratory trees, and Cuvierian organs; hermaphrodite; e. g. Synapta, Chirodota.
Holothurioidea, Theel, Challenger Reports, xiv. 1886. Elasipoda, Id. ibid. iv. 1882. Variations in Holothurians, Lampert, Biol. Centralblatt, v. 1885-6.
Histology, Hamann, Beitrage, etc, 'Die Holothurien,'Jena, 1884; Id. Z.W. Z. xxxix. 1883; Jourdain, Annales Mus. Nat. Hist. Marseilles, i. 1883.
Pharynx of unknown Holothurian with calcareous plates, Moseley, Q. J. M. xxiv. 1884. Cotton Spinner (Holothuria nigra) ejecting Cuvierian organs, Bell, P. Z. S. 1884.
Haemoglobin in Holothurian (Thyonella gemmata), Howell, Studies Biol. Lab., Johns Hopkins Univ., iii. (6), 1886.
Ciliated funnels of Apneumona, Semper, 'Reisen im Archipel der Philippinen,' i. 1868, p. 32.