The holothurians, or sea-cucumbers, although in appearance quite unlike starfishes and sea-urchins, have the characteristic ambulacral zones and other features of the group. In form they are cylindrical, and, when the tentacles and tube-feet are retracted, resemble fat worms; when fully expanded they are somewhat like sea-anemones, the tentacles forming a rosette-like top. The walls of the body are tough and muscular, with small calcareous deposits or spicules of various shapes in the skin. The mouth is at one end, the excretory opening at the other, and along the body are double rows of tube-feet. Often instead of tube-feet, or together with them, are conical processes without suckers. The ambulacra, when arranged in regular zones, are used for locomotion only in the lines running from the madreporic plate. In some species three of the zones are near together, and form a kind of sole on which the animal creeps; again the tube-feet are wholly suppressed, as in /Synapta/. Besides progressing by means of these suckers, the holothurians move, as do worms, by the extension and contraction of the body. The inner surface of the tough membrane inclosing the body is lined with powerful longitudinal and transverse muscles, by means of which the creature contracts and lengthens its body and changes its form in a wonderful manner. Around the mouth are tentacles, which are often much branched and are used as organs of touch and smell, and sometimes have an ear-sac at the base. From the mouth the food-canal, making one long coil, extends to a chamber [pg231] (/cloaca/) at the other pole. The cloaca gives off a pair of much-branched respiratory trees, which are constantly supplied with water by the contractions of the cloaca. At the base of one of the respiratory trees are singular structures known as /Cuvierian organs/. They are numerous, viscid, glandular tubes, which the animal can throw out, and which will adhere closely to almost anything. The holothurian has a water-vascular system, the madreporic plate being near the mouth, but not opening to the outside, and a nervous system which starts from a ring which lies around the mouth. The egg-sacs are branched tubes, often highly colored, which open to the outside, close to the wreath of tentacles surrounding the mouth.