In the cut to the left _1_ first advances, then (_5_) and (_2_); _3_ and _4_ remaining at rest. Whereupon (_5_) and (_2_) simultaneously come back to the positions _5_ and _2_, _c_ is lifted and pushed forwards with _1_, while the two rays _3_ and _4_ are pulled along behind. In the figure to the right the same animal first shoves forward the pairs (_1_) (_2_) and (_5_) (_3_), _4_ remaining at rest, and then bends both pairs backwards, dragging only _4_ behind; _c_ is lifted and thrown forward in the direction of the arrow.] While in this instance, accordingly, the arms, although they are not immovable and bend and approach each other, officiate rather as the bearers of organs than as prehensile and locomotory apparatuses themselves,—the sucker-feet performing the principal tasks and requiring for their work a very finely ramified nervous system; in the case of a certain other division of the star-fishes, the so-called sand-stars (_Ophiuridae_), the arms are thinner and more supple, and act as organs of prehension and locomotion, dispensing more or less entirely with their suctorial pedicels. By alternately thrusting three feet forward (Fig. 3) and then drawing back the two side feet of these three, the five-footed sea-stars move more swiftly than the others, sometimes proceeding by jumps even; but they cannot climb up smooth surfaces, or cliffs, unless irregularities are present which may be grasped by their pliant arms, whereas on the other hand the common star-fishes, which are furnished with sucker-feet, climb best of all on smooth and slippery surfaces, each one of their countless pedicels being able to suspend a considerable weight, in some species as much as twenty-five grammes. In other respects, especially with regard to the ring-shaped connection of the five nerve-cords, their organisation is essentially the same; only in the sand-stars the central portion forms a disc more distinctly separate from the arms, in which former the common organs of feeling and digestion have more fully retracted.