How does the Starfish know where it is going? Underneath each ray, near the tip, is a little feeler (or tentacle) and a little eye spot. By means of these it gets an idea where each ray is going to; and, since it often moves but one ray at a time, this is sufficient for it. When necessary, however, the several rays can act in concert with one another. The rayed form of the Starfishes led to their being at first included in the group of Radiate Animals, along with the tentacle-bearing Coelenterata; but it has long been recognised that they are animals of much higher structure. Their very larvæ can barely be brought into comparison with animals so simple as the true "radiates." The Snake-Stars, or Ophiuroidea, are closely allied to the Starfishes. In these the arms are thin and sharply defined from the little central disc, instead of sloping gently out of it, as in the Starfishes. The rapid wriggling movements of the arms have gained for them their very appropriate name. They are also called Brittle Stars, because the arms break off easily, sometimes at the will of the animal. Several kinds of them are common on our shores, although they are not so common as the ordinary Starfishes. Fig. 37 shows the general form of a Brittle Star. [Illustration: FIG. 37.--A Brittle-Star, _Ophiopteris antipodum_.] [Illustration: 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