In a living Star-fish the tube-feet or pedicels already mentioned are seen projecting from each side of the ambulacral groove; and, with the exception of a few at the tip of each arm, all the tube-feet terminate in a well-formed sucker, by means of which they can be firmly fixed to a flat surface (Fig. 35). If we wish to understand the structure and mechanism of this locomotor or ambulacral system--which, I may observe in passing, is of special interest from the fact that as a mechanism it is unique in the animal kingdom--we must resort to dissection. We then find that each of the tube-feet is provided in its membraneous walls with a number of annular or ring-shaped muscular fibres; when these fibres contract, the fluid contained in the tube is forced back, while, conversely, when these fibres relax, the fluid runs into the tube. If the contraction of these fibres is strong, the tube shrinks up entirely, _i.e._ is retracted within the body of the animal; but if the contraction of the fibres is not so strong, the tube is only shortened. If, before its shortening, its terminal expansion, or sucker, has been applied to any flat surface, the effect of the shortening is to cause the sucker to adhere to the flat surface, in consequence of the pressure of the surrounding sea-water being greater than that of the fluid within the shortened tube. In this way, by alternately contracting and relaxing the muscular fibres in the walls of a tube-foot, a Star-fish is able alternately to cause the terminal sucker to fasten upon and to leave go of any flat surface upon which the animal may be crawling. In other words, when the tube-foot is about to form its attachment to a flat surface, it is fully distended with fluid; but when the terminal sucker touches the flat surface, this fluid is partly withdrawn, so causing the sucker to adhere. [Illustration: Fig. 36.--Diagram of ambulacral system of a Star-fish: _a_, madreporic canal; _b_, inner end; _g_, outer end of sinus leading to circular neural vessel; _h_, from which radial neural vessels, _l_, arise; _c d_, Polian vesicles; _f_, ampullæ; _m_, oral aperture; _n_, madreporic plate.]