Note that the ampullæ and tube-feet are all connected with the radial canals. By a contraction of the delicate muscles in the walls of the ampullæ the fluid in the cavity is compressed, thereby forcing the tube-feet out. By the contraction of muscles in the tube-feet they are again shortened while the small disk-like terminal sucker clings to some firm object. In this way the animal pulls itself along by successive "steps." This entire system, called the _water-vascular system_, is characteristic of the branch Echinodermata. In addition to the fluid in the water-vascular system there is yet another body-fluid, the _perivisceral fluid_, which bathes all of the tissues and fills the body-cavity. [Illustration: FIG. 19.--Semi-diagrammatic figure of cross-section of the ray of a starfish, _Asterias_ sp.] TECHNICAL NOTE.--Take a drop of the perivisceral fluid from a living starfish and examine under high power of microscope, noting the amœboid cells it contains. The perivisceral fluid is aerated through outpocketings of the thin body-wall which extend outward between the calcareous plates of the body. These outpocketings have already been mentioned as the respiratory cæca (see p. 109). Surrounding the stone canal is a thin membranous tube, and within it and by the side of the stone canal is a soft tubular sac. The function of these organs is not certainly known. Work out the _nervous system_; note, as its principal parts, a nerve-ring about the mouth, and nerves running from this ring beneath the radial canals along each arm. =Life-history and habits.=--The starfishes are all marine forms. They hatch from eggs, and in their early stages are very different in appearance from the adults. At first they are bilaterally symmetrical, their radial symmetry being acquired later. Thousands of eggs and sperm-cells are extruded into the sea-water, where fertilization and development take place. The young swim freely in the open sea, feeding on microscopic organisms, and then undergo very radical changes in the course of their development. The adults are for the most part carnivorous, feeding on crabs, snails, and the like. The live prey is surrounded by the extruded stomach which secretes fluids that kill it, after which the soft parts are digested. (See general account of the life-history of Echinoderms on p. 119.) THE SEA-URCHIN (_Strongylocentrotus_ sp.) =External structure.=--TECHNICAL NOTE.--If fresh or alcoholic specimens or even the dry "tests" of the sea-urchin (fig. 20) are to be had, the general characteristics of the external structure can be made out. How does the external surface of the sea-urchin differ from that of the starfish? Can you find the very long _tube-feet_? Where is the mouth-opening? With what is it surrounded? Each tooth is enclosed in a calcareous framework. The whole structure is known as "_Aristotle's lantern_."