The echinoderms are radially symmetrical, i.e. a number of similar parts which together make up the body are arranged around an axis (Fig. 145). These animals are usually five-rayed. The mouth lies at lower end of the axis of the body. In some echinoderms the radial symmetry is indistinct, and at the same time there is a more or less clear bilateral symmetry (p. 16). The dermal skeleton either consists of numerous microscopic calcareous deposits, or of firm plates; the skin often bears spines, hence the name of the group. Echinoderms move by means of rows of tubular feet, which are connected with a water-vascular system, and are protruded by means of small bladders, connected with the same system, which contract and force water into them. The form of the body is apple-shaped to spherical (Regular Sea Urchins), shield- or heart-shaped (Irregular Sea Urchins), pentagonal to star-shaped, with arms of varying length (Starfishes), or cylindrical to vermiform (Sea Cucumbers). All echinoderms are marine, and none of them, therefore, are of agricultural importance, though near the coast starfishes are sometimes used as manure.