The muscular system, as already stated, is almost always present in the Echinodermata, but the organs of locomotion are very various, the principal being the membranous tubes usually termed feet, or ambulacra, which issue from the ambulacral apertures; but besides these, the rays themselves are movable, and in animals which are free to move from place to place these are used for the purpose. Thus in the common star-fish the rays may be bent towards the upper or lower surface of the disk, so as to facilitate its advance either in water over small spaces or up the vertical face of rocks. These ambulacra are very numerous, disposed in rows along the under surface of the rays; thus in _A. aurantiaca_ there are two simple rows of feet attached to each ray, and the vesicular part is deeply cleft into two lobes; while in _A. rubens_ (Fig. 105) there are two double rows on each ray, and each foot has an undivided vesicle. Each of these ambulacra consists of two parts, an internal and generally vesicular portion placed within the body, and a tubular portion outside, projecting from the surface through an aperture in the skin or shell, the tube being closed at the extremity, and terminating in a sucker, usually in the form of a disk slightly depressed in the centre. The feet are thus muscular fleshy cylinders, hollow in the centre, and very extensible; by means of them the animal draws itself forward. The foot is extended by the contraction of its internal vesicle, which forces the fluid into the hollow tube, or, where the vesicle is wanting, by projecting the fluid into the tube by a communicating vessel. The tubular part is thus distended and elongated, and again retracts itself by means of its muscular fibres, by which action the fluid is forced back into the interior. In progression the animal extends a few of its feet, attaches its suckers to the rocks or stones, then, by shortening its feet, it draws its body forward. The progression of the Asterias is thus very slow, and so regular that only the closest observation enables the spectator to discover the movement which produces it. Like the movements of the hands of a watch, the eye cannot quite follow it. When an obstacle presents itself--if, for example, a stone comes in its way--it raises one of the rays in order to obtain a point of support, then a second ray, and, if necessary, a third,--and thus the animal creeps over the stone with as much ease as if it walked over the smooth sands. In the same way the animal creeps up perpendicular rocks, which is accomplished by means of these ambulacra and suckers. Frédol says: "If an Asteria is turned upon its back it will at first remain immovable, with its feet shut up. Soon, however, out come the feet, like so many little feelers; it moves them backward and forward, as if feeling for the ground; it soon inclines them towards the bottom of the vase, and fixes them one after the other. When it has a sufficient