The rectum, or cloaca, is one of the most characteristic features in this and most other Sea-cucumbers. In addition to the passing of faeces, it is used to pump water in and out, and it thus serves as a breathing organ. This pumping is effected by alternate contractions of the radiating muscles attaching the cloaca to the body-wall, and of the circular muscles which immediately surround it. Two long branched tubes termed Respiratory trees (Fig. 256, 11) open into the cloaca, and into these the inspired water penetrates. The finer branches of these gills end in rounded thin-walled swellings termed "ampullae"; and when water is forced into these they become tense, and a considerable quantity diffuses through their walls, carrying oxygen into the fluid which fills the coelom. If a Sea-cucumber be left in a limited quantity of water, it will sometimes direct the posterior end upwards until it reaches the surface of the liquid, and will pump air into the trees. Besides the trees, other much shorter tubes open into the cloaca, termed the Cuvierian organs. These tubes are really the modified basal branches of the trees. They are unbranched, and their peritoneum consists of cells which secrete a slime which swells up enormously on the addition of sea water. When the Cotton-spinner is strongly irritated, it contracts all the muscles of the body-wall, and these, acting on the incompressible fluid in the body-cavity, transmit the pressure to the thin rectum, which tears, and allows a portion of the viscera to be forced out. The first parts to be rejected are the Cuvierian organs, and the cells covering these absorb water, and their contained mucus splits up into a tangle of white threads, in which an enemy may be completely ensnared. A large lobster has been seen so enveloped with this "cotton" as to be completely incapable of motion. The origin of the name "Cotton-spinner" requires no further elucidation. Such self-mutilation, even when it involves not only the Cuvierian organs, but the trees and the whole of the intestine, is not necessarily fatal. If the animal be left alone, it can regenerate the whole of these organs.
The water-vascular system in its general features resembles that of the Echinoidea. We notice as its first striking peculiarity the modification of the stone-canal. This is often multiplied, as in the species (H. tubulosa) represented in Fig. 256, where there are five; but whether there is one or many, they do not reach the body-wall, but end each in a swelling projecting into and bathed by the coelomic fluid. These swellings are termed "internal madreporites." They are pierced by numerous fine ciliated canals, which lead into a space from which the stone-canal takes its origin. Both stone-canal and madreporite (especially the latter) are stiffened by the deposition of carbonate of lime. In the young Holothurian there is a single ciliated pore-canal opening to the exterior and leading into a thin-walled axial sinus, which, as Bury[[505]] has shown, is later converted into the internal madreporite; the pore-canal, which represents the external madreporite of other Echinoderms, disappearing at the same time.
Fig. 256.—Dissection of Holothuria tubulosa. × ½. 1, Feelers; 2, feeler-ampullae; 3, ring-canal; 4, Polian vesicle; 5, stone-canals; 6, radial canal; 7, one of a pair of longitudinal muscles; 8, genital tubes; 9, intestine; 10, radiating muscles of cloaca; 11, base of respiratory tree; 12, ventral blood-vessel; 13, plexus of dorsal blood-vessel. (After Ludwig.)
This extraordinary modification is the consequence of the habit of forcing water into the respiratory trees. The body-cavity is by this means kept tensely filled with fluid, and the stone-canal is enabled to draw on it for the supply to the water-vascular system, thus rendering the external madreporite supererogatory. A large-stalked sac—the Polian vesicle (Fig. 256, 4)—multiplied in many species, hangs down from the water-vascular ring and serves as a reservoir of fluid.
All the podia, including the feelers, have ampullae. In the feelers a semicircular valve is situated just where the external part passes into its long ampulla. When this valve is expanded, the feeler is moved about by the contraction of its muscles, but when it is contracted, the contents of the feeler can flow back into the ampulla, so that the feeler is reduced to an insignificant papilla (as in Fig. 254). The interior of the feeler is ciliated, and a current seems to flow up one side and down the other, so that this organ, like the dorsal tube-foot of a Cake-urchin or Heart-urchin, seems to assist in respiration.
The nervous system differs from that of Echinoidea in the absence of the pigment spot (or so-called eye) on the terminal podium of the radial water-vascular canal. Each podium receives a so-called nerve—really an extension of the radial nerve-cord with its ganglion-cells—and this ends in a plate of sensory epithelium in the sucker of the tube-foot or tip of the tentacle, or of each of its branches in the case of the feeler.
There is a coelomic nervous system developed from the radial perihaemal canals. The perihaemal ring is represented in Echinoidea by the lantern coelom, in Holothuroidea in all probability by the "buccal sinus," a space intervening between the water-vascular ring and the oesophagus. In the outer wall of this are developed ossicles, which constitute the calcareous ring found in all[[506]] Sea-cucumbers (Fig. 257, A and B). In this ring (Fig. 257, B) are to be distinguished radial and interradial pieces. The former are notched at their upper ends, and in all probability represent the auriculae of Echinoidea, as the radial nerve-cords pass out over the notches, whilst the interradial pieces probably represent a coalesced pair of jaws and their included tooth, since these ossicles develop from a single rudiment in the larval Echinoid.
The so-called blood system is in its main features similar to that of Echinoidea. It consists of a blood-ring surrounding the oesophagus inside the water-vascular ring, and sending branches along the stone-canal, and of dorsal and ventral strands accompanying the gut in its course. These are best marked in the region of the intestine, where absorption principally takes place; in the wall of the stomach they are represented by a delicate plexus which can hardly be traced into connexion with the blood-ring. The dorsal "vessel" is situated in a fold of peritoneum projecting from the intestinal wall; it gives off branches to the intestine, which unite on its surface to form a plexus. In the middle limb of the intestine these branches are grouped into tufts, and the fold of peritoneum between successive tufts becomes absorbed; through the holes so formed branches of the respiratory tree penetrate, so that the trees cannot be separated from the intestine without tearing the dorsal vessel (Fig. 256, 13).
