The tube-feet of Brittle Stars are very small and are not provided with suckers, but are very sensitive, serving the purpose of feelers; also, having thin, permeable walls, they probably play a large part in the process of respiration. Both arms and disc are hardened by a dense scaffolding of calcareous plates; and not only are the former attached to the latter by a well-formed joint, but the arms themselves are constructed of a number of segments that are held together by a kind of ‘tongue and groove’ joint. Round the mouth are a number of tentacles that are kept in constant motion with the object of carrying the food towards it, and of holding the larger morsels while the act of swallowing is progressing. [Illustration: FIG. 107.--THE COMMON BRITTLE STAR] The various species of Brittle Stars live among the rocks and weeds, chiefly in deep water, where they move about by means of the muscular contraction of their arms, the disc being raised on the curved arms as the animal proceeds. Some species are to be found between the tide-marks, and especially abundant on the south-west coast are two small species that live among the tufts of coralline weeds, sometimes so crowded together that dozens may be taken from a little patch of coralline only two or three inches square. These have such small discs, and such slender arms, and are, moreover, so well concealed by their colouring, which closely resembles that of the weed-tuft they inhabit, that they are only to be detected by close inspection. The remaining division of the starfishes, sometimes distinguished by the name of Common Stars, possess five arms or rays, which may be either long or short, and which are not jointed with the central disc, but continuous with it; that is, there is no sharp line of demarcation between arm and disc. One or two species are well known to all frequenters of the sea-side, but the majority of them inhabit deep water, where they creep about over the rocks and weeds, obtaining their food from the bed below them. If we examine the common five-finger star that is so often stranded on the beach, and so frequently found in rock pools between the tide-marks, we see that each arm has a large and conspicuous groove running along its centre on the under side, and on each side of these are the rows of tube-feet, arranged in such a manner that they have suggested the appearance of an avenue of trees on each side of a garden walk, and have consequently earned the name of _ambulacrum_. These tube-feet may be protruded for some distance; and, being provided with suckers that possess considerable clinging power, they form the principal means of locomotion. Put the starfish in the aquarium, or in a tidepool by the sea, and you will find it very interesting to observe how the animal progresses, while some idea of the clinging power of the tube-feet may be ascertained by allowing the animal to creep over the submerged hand.