As all of the Echinoderms except some of the feather-stars can move about, they have organs of locomotion, and well-defined muscles for the movement of the locomotory organs. The external organs of locomotion, the tube-feet (in the sea-urchins the dermal spines aid also in locomotion), are parts of a peculiar system of organs characteristic of the Echinoderms, called the ambulacral or the water-vascular system. This system is composed of a series of radial tubular vessels which rise from a central circular or ring vessel and which give off branches to each of the tube-feet. The water from the outside enters the ambulacral system through a special opening, the madreporic opening, and flowing to the tube-feet helps extend them. The tube-feet usually have a tiny sucking disk at the tip, and by means of them the Echinoderm can cling very firmly to rocks. =Development and life-history.=--Differing from the sponges and the polyps and jellyfishes, the reproduction of the Echinoderms is always sexual; young or new individuals are never produced by budding, or in any other asexual way. The new individual is always developed from an egg produced by a female and fertilized by the sperm of a male. The eggs are usually red or yellow, are very small (about 1/50 in. in diameter in certain starfishes), and are fertilized by the sperm-cells of the males after leaving the body of the female. That is, both sperm-cells and unfertilized egg-cells are poured out into the water by the adults, and the motile sperm-cells in some way find and fertilize the egg-cells. From the egg there hatches a tiny larva which does not at all resemble the parent starfish or sea-urchin. It is an active free-swimming creature, more or less ellipsoidal in shape and provided with cilia for swimming. Soon its body changes form and assumes a very curious shape with prominent projections. The larvæ of the various kinds of Echinoderms, as the starfishes, sea-urchins, sea-cucumbers, etc., are of different characteristic shapes. The naturalists who first discovered these odd little animals did not associate them in their minds with the very differently shaped starfishes and sea-urchins, but believed them new kinds of fully developed marine animals, and gave them names. Thus the larvæ of the starfishes were called Bipinnaria, the larvæ of the sea-urchins Pluteus, and so on. These names are still used to designate the larvæ, but with the knowledge that Bipinnaria are simply young starfishes, and that a Pluteus is simply a young sea-urchin. From these larval stages the adult or fully developed starfish or sea-urchin develops by very great changes or metamorphoses. The Echinoderms have in their life-history a metamorphosis as striking as the butterflies and moths, which are crawling worm-like caterpillars in their young or larval condition.