The surface of the body is divided definitely by the ambulacra, the /ambulacral zones/, and the /interambulacral spaces/, but differently in the different classes. The movement of the animals is [pg203] effected by means of the tube-feet, or ambulacra. These have suckers, which are attached or released by the power of the water-vascular system. The /madreporic plate/, which can be distinctly seen in starfishes at the angle of two of the arms, is a calcareous porous plate which opens into the /stone-canal/. This canal passes through the body and opens into a circular tube which runs around the mouth. A system of canals extends from this circumoral tube, or /ring-canal/, and eventually one canal opens into each tube-foot. Water, entering by the madreporic plate, passes through the various canals and into the tube-feet, which it distends. When the feet are pressed against a substance and the water is withdrawn, a vacuum is formed, which causes the disk-ends of the feet to act like suckers. When the feet are again flooded the hold is relaxed. Thus the animal drags its body along with a slow, gliding motion. The madreporic plate, being calcareous, is said to act as a filter, purifying the water as it passes into the body, which it furnishes with oxygen as well as with locomotive power. [Illustration: Aristotle's lantern.] A curious organ found in the /Echinoidea/ (sea-urchins) is called /Aristotle's lantern/. It is a complicated arrangement, not fully understood, having forty parts, and is connected with the alimentary system. One of the parts is the mouth, over which five teeth project. These teeth grasp and grind the food. The exoskeleton is composed of plates of carbonate of lime, called /ossicles/, which cover the surface of the body. The ossicles are of various sizes and shapes, and are one of the determining features in classification. They may be scattered spicules of lime, separated plates joined by connecting rods, or overlapping plates; or they may be fitted together so as to form a continuous shell. The starfishes are usually carnivorous, the sea-urchins usually vegetarian, while the holothurians, after the manner of worms, take in sand and mud, deriving their nourishment from the organic particles contained in them. All echinoderms live in the sea. [pg204] TABLE SHOWING THE CLASSIFICATION OF THE STARFISHES DESCRIBED IN THIS CHAPTER. Class =ASTEROIDEA= (/Starfishes/) Order =PHANEROZONIA= (/Ossicles not crowded; large marginal ossicles; spines only on dorsal surface; pedicellariæ sessile/) Families Genera Species ~PORCELLANASTERIDÆ~ ^Ctenodiscus^ ^C. corniculatus^