Fig. 38. A Sea-Cucumber, Cucumaria Planci, from Naples, natural size.
The Sea-Cucumbers, Holothuroidea, are another group of Echinodermata that are represented on our own coasts; by small specimens, however, while the Pacific Ocean furnishes instances of larger size—the Trepangs—which are used by the Chinese as articles of food. The name Sea-Cucumber is given in fanciful comparison to a small Gherkin; presumably one that has been very badly pickled—for the colour of the animal is brownish and by no means green. The mouth of a Sea-Cucumber is surrounded by a circlet of tentacles (partially indicated in the diagram, [Fig. 38]). The body is elongated and crawls along: the "star" shape, so characteristic of the Echinoderms, is scarcely to be recognised except in cross section, where the longitudinal rows of tube-feet are seen to outline a pentagon. The skeleton of the Sea-Cucumber is of a very meagre description. Instead of forming a rounded case, as in the Sea-Urchin, it consists only of loose pieces of very small size, situated below the skin. The Starfishes are intermediate in this respect. Their "skeleton" consists of a vast number of pieces or "ossicles," which are of fair size, but are not closely united, as in the Sea-Urchin. They are, however, so numerous and so well knit, that the skeleton of a dead Starfish presents the complete outward form of the animal. It must be noted that the ordinary skeleton of the Sea-Urchin is only apparently exterior. As is the case with the ossicles of the Starfish and Sea-Cucumber, the skin lies outside, and the hard particles belong to the middle layer, or mesoderm. In this the skeleton of Echinoderms differs from the "shell" of a crab or lobster, which is formed by a hardening of the skin itself.
Fig. 39.—A, Head of a Stone Lily or Encrinite, Encrinus liliformis, a fossil from the Muschelkalk of Brunswick, natural size. B, Rock with stalks of encrinites. C, Section of a stalk.
The Crinoidea, Encrinites or Stone-Lilies, form another group of the Echinodermata. Though still represented by living forms, they attained their maximum development in past ages. The English "Mountain Limestone" of the Carboniferous period is full of their fossilized remains, which form a marble often used for ornamental purposes. The so-called "Stone Lily" consists of a "head" comparable with the body of a Star-fish or other Echinoderm, which is borne at the end of a long fixed stalk. The marble above named owes its ornamental appearance to the presence of these stalks, often very long, and cut through at every possible angle. The Crinoids have their living representative in English Seas, Antedon, the Feather-Star ([Fig. 40]). On the side opposite the mouth, where, in the Encrinite, the stalk would be, there are a group of elongated processes called cirrhi, by means of which the animal can attach itself to stones or seaweeds. When not thus fixed, it swims about, by moving its fringed arms, each of which is forked. It will be seen that when the animal is fixed by its cirrhi, it stands mouth upwards, so that its position compared with that of the Starfish or Sea-Urchin is upside down. The young of the Feather-Stars have stalks by which they are fixed, like the Encrinites; but afterwards the stalk is lost.
Fig. 40.—A Feather-Star, Antedon bifida, British Seas, three-quarters of the natural size. The short threads in the middle are the cirrhi.