178 LOWER INVERTEBRATES. 



hcnuMphrorlite. In the majorit}^ of the class the sexes are flistiiict. The oviduct opens 

 near the mouth. Tlie ovum, after segmentation, becomes, by tlie invagination or turn- 

 ing inwards of a part of the external surface, converted into a. hollow gastrula, the 

 opening of which becomes the anus, while a mouth and gullet are produced by the 

 invagination of the outer layer of tissue or ectoderm. The completed alimentary canal 

 of the larva consists of a gullet, a rounded stomach, and an intestine, and the cilia of 

 the external surface become restricted to a number of hoops or bands, from one to five 

 in number, bent upon themselves, yet passing all round the body. These are accom- 

 j)anied by certain ear-like projections, from which the young is known as an Auricidaria. 

 Before the auricularia is fully formed the young holothurian buds out near the side of 

 the stomach, and gradually develops its spicules. The ear-like processes disappear, the 

 auricularia becomes cylindrical, the body of the embrj'o elongates, tentacles are devel- 

 ojied ai-ound the mouth, and the young holothurian is complete. The entire course of 

 development is largely parallel to that of the other echinpderms, but the holothurian 

 is more directly developed from the larva than is the case in sea-urchins and star- 

 fish. Some holothurians, like some star-fishes and sea-urchins, have the larval 

 stages suppressed or only slightly indicated, the young develojiing in a sort of 

 marsujjium. 



The holothurians arc of little economic value, and with us arc regarded merely as 

 objects of scientific interest. In the East they are more important, and as 'trepang* 

 play a prominent part in the diet of the Chinese and other oriental peoples. The 

 trade of preparing the trepang is almost entirely in the hands of the Malays, and every 

 year large fleets set sail from Macassar and the Philippines to the south seas to catch 

 the ' Beche-de-Mcr.' They are split open, boiled, dried in the sun, and then smoked 

 and packed in bags. The annual catch is estimated at about four hundred tons, and 

 the price varies according to quality from seven to fifty cents a pound. Trepang is 

 very gelatinous, and is used as an ingredient in soups. 



Although the holothurians must evidently be classed with the Echinoderms, their 

 simjilest forms, as Eupt/rgiis, present nothing of the radiate arrangement except the 

 circle of tentacles ; and their affinities to such worms as /Sijnmculus and its allies, 

 which have also a com])lete circle of tentacles, a ring-canal, and a kind of water-system, 

 seem in many respects close. 



The Holothuroidea have been said to feed on living coral, but this seems disproved 

 by recent observations. The manner in which they feed is well illustrated by the 

 following account of the habits of a species of Cuctcmaria, common upon the coast of 

 Cornwall. When in full feed the tentacles were observed to be in constant motion, 

 each separate tree-like plume, after a brief extension, being inverted and thrust bodily 

 nearly to its base in the cavity of the pharynx, bearing with it such fi-agments of sand 

 and shelly matter as it had succeeded in grasping. No particular order was followed, 

 but the meal continued for hours. One might imagine a child with ten arms, like 

 an ancient Buddha, grasjjing its food with every hand, and thrusting hand and arm 

 down the gullet with each handful. These animals were kept in a tank with living 

 corals without in any way interfering with them. The nutriment must be furnished 

 liy the Infusoria, diatoms, and other microscojiic animals and vegetables which 

 always more or less cover the debris at the bottom of the water. Probably the 

 shell or coral debris is triturated by the teeth of the pharynx. It has been calculated 

 that fifteen or sixteen of these creatures will remove about eighteen cubic feet of coral 

 per annum. 



