There are several other species of Sea-Urchins whose forms are tolerably well distinguished by their popular appellations. Thus we have the 'Silky Spined Urchin;' the 'Green Pea-Urchin,'--the latter is the commonest and prettiest of all its kindred, its back being covered with a kind of powdery green, as is seen on the elytra of many beetles; the 'Cake-Urchin,' which from its flattened form may be regarded as a link between the Sea-Urchins and the true Star-fishes; the 'Purple Heart-Urchin,' and the pretty 'Rosy Heart-Urchin,' appropriately named from the brilliant crimson hue that its body presents during life. CHAPTER XX. Sea-Cucumbers. (HOLOTHURIADÆ.) XX. There is a very singular group of animals, the _Holothuriadæ_, that claims a passing notice, from their near relation in structural formation to the Sea-Urchins, although externally they also exhibit a certain resemblance to the _Annelides_. They are commonly termed Sea-Cucumbers, from the fancied likeness which they bear, both in shape and colour, to their namesakes of the vegetable kingdom. A Holothuria is very unattractive in appearance when lying listless upon the sea-beach, but if a small specimen be transferred to the aquarium, it exhibits features of a very singular and interesting character. When about to change its position, the head, hitherto concealed, is protruded and expanded, until it assumes the form of a beautiful flower. The animal moves principally by aid of sucker-like feet, similar in form to those of the Asteriadæ, or Sea-Urchins. In most species, the body is divided longitudinally into five rows of suckers. In some, however, these organs are scattered over the entire surface, while in the small Sea-Cucumber (_Psolus phantapus_), they are arranged in three rows upon a soft, oblong, flat disc, situated beneath the body of the animal, like the foot of a gasteropod mollusc. Of one genus--the Trepang--many species are eaten by the omnivorous inhabitants of the Celestial Empire, by whom it is employed in the preparation of 'nutritious soups, in common with an esculent sea-weed, shark's fins, edible birds' nests, and other materials affording much jelly.' The intestines, which are generally found to be filled with coral, and solid masses of madreporic rock, are extracted, and the animal then boiled in sea-water and dried in smoke. Nothing can possibly be less enticing than the black and shrivelled carcases of these defunct gasteropods, as they are seen spread out and exposed for sale in the China markets. There are many varieties of Trepang, some being held in higher esteem than others,--hence the great difference which exists in the price of the article. The lowest quality being ten dollars, and the highest fifty dollars, per pecul of 133 lbs. The following are titles by which a few of the Holothuriæ are known in China:--