272 



NATURAL HISTORY. 



within the test of an Urchin, and also serve the same purpose, viz., the attachment of muscles. 

 These organs are disposed in five bands, which correspond in position with the radial nerves 

 proceeding from the oral ring. The mouth is at one end of the body, and the gullet leads into 

 a long and much coiled digestive tube of tolerably uniform width, which terminates in a large 

 poxich or cloaca, at or near the opposite extremity. 



Around the mouth is a fringe of branched tentacles (Fig. 17) connected with the water- 

 vascular ring. In a few species this ring communicates directly with the exterior by means of a 



water-tube opening upon the surface of the body. But in most 

 Holothurians the water-tube hangs down freely into the body-cavity, 

 and terminates in a sieve-like madreporite. One or more Polian 

 vesicles are attached to the water-vascular ring in the intervals 

 between the origins of the radial vessels, with which tube-feet pro- 

 vided with ampulla? are connected. In some forms (Cucumaria) * these 

 tube-feet are evenly distributed, and almost equally developed on all 

 the radial vessels (Fig. 17) ; but in others (Psohis) they are confined 

 to three out of the five vessels, that are arranged in a flat sole-like 

 disc, on which the animal creeps. In the Elasmopoda the two lateral 

 vessels of this under surface are the only ones in the body, the 

 three remaining vessels being suppressed ; while in Synapta^ and its 

 allies there are no radial vessels at all, the oral ring and the tentacles 

 connected with it being the sole representatives of the water-vascular 

 system. 



The blood-vascular system consists essentially of dorsal and ventral 

 vessels along the digestive tube, as in the Urchins. These are connected 

 with an oral plexus, from which the radial blood-vessels originate. But 

 no representative of a " central plexus " has yet been made out, except 

 in the Elasmopoda, in which the two extremities of the dorsal vessel 

 are united by a large contractile trunk. 



Respiration is largely effected by the branched tentacles round the 

 mouth, which are connected with the water-vascular ring. The network of vessels on the walls of 

 the digestive tube seems to take part in the same work, water entering the intestine from the cloacal 

 pouch, which is capable of expansion and contraction. Connected with it in some Holothurians are 

 two branched tubular organs, the " respiratory trees " or lungs, through which water can pass into the 

 body-cavity by fine pores at the ends of the branches. The left lung may be in close relation with 

 the vessels of the dorsal intestinal plexus. 



The Holothurians may attain a considerable relative size, some of them being a foot long, and 

 capable of extending to thrice that length. Locomotion is largely effected by the extension and 

 contraction of their bodies, which are continually changing their form by the action of strong muscles, 

 both longitudinal and transverse, Sometimes, indeed, the contractions are so forcible that the 

 creature throws out all its viscera through the cloaca, and lives for a time without them, until it can 

 make good the loss by growing a new set. 



The English Holothurians live among seaweeds or in sand or mud, with the body concealed 

 and the tentacles exposed. They take a great deal of sand into their digestive tube, and the intestines 

 of those which live in the neighbourhood of coral reefs generally contain fragments of coral. When 

 the nutritious matter has been extracted from the coral or sand, the latter is passed out through 

 the cloaca. 



The Trepangs of the tropical seas form an important article of food in China. About thirty-five 

 different varieties are enumerated by the Chinese traders, but only about five have any great 

 commercial value. In Fiji they are accounted " royal fish," and used only to be caught by command 

 of the supreme chief. Enough " fish " to fill a three-bushel bag, when dried, may be caught in two 

 nights. The value of such a bagful would be from twenty-five to forty shillings, according to variety 

 and the perfection with which it is cured. The process is effected as follows : The viscera are 

 * Latin, cucumis, a cucumber. t Greek, synaptos, joined together. 



Fig. 17. HOLOTHURIAN (CUCU- 



tnaria planci) WITH ITS BUCCAL 

 TENTACLES EXPANDED. TWICE 

 NATURAL SIZE. 



