SEA-URCHINS, CAKE-URCHINS, HEART-URCHINS ORDER =DESMOSTICHA= SEA-URCHINS The shell of a sea-urchin consists of many calcareous plates, or /ossicles/, fitted closely together and forming a continuous exoskeleton. The plates are so united that externally the marks of juncture are not perceptible, but on the interior the shape of these plates is well defined. In examining such a shell, or test (they abound on the beach), it will be seen that its surface is covered with numerous hemispherical projections or knobs, which are grouped in double rows and run in meridional lines from one pole to the other of the more or less spherical body, separating it into ten divisions. Five of these divisions have perforations, or small pores in the plates of the shell, and are called the /ambulacral zones/ or areas, because through these pores pass the small tubes, in the living animal, which connect the tube-feet, or /ambulacra/, with the radial water-canals and the /ampullæ/ (see page 206). The wide spaces between these double rows of pores are called the /interambulacral zones/ or areas. The ten spaces diverge from the /peristome/, or soft part around the mouth, in the center of the lower surface, and converge in the small area at the top or aboral side. In the center of this small circular dorsal space is the excretory opening, and surrounding it are ten plates, five of which have openings into the egg-sacs. One of them is larger than the others, and is modified to form the /madreporic plate/. The other five plates have eye-specks. The ambulacral zones terminate at these ocular plates. The numerous spines which cover the animal are of three [pg219] kinds, and proceed from the knobs on the exoskeleton, over which they fit, forming ball-and-socket joints, which enable them to move in any direction. The long spines are ribbed, and seem to have no other function than that of protection. The second set, the /pedicellariæ/, are very peculiar small organs scattered over the surface in great numbers, and consisting of a head bearing three bill-like blades mounted on a long, flexible stalk. The office of some of the pedicellariæ is to remove waste from the excretory opening; this is passed down regular lines and dropped into the water, thus keeping the body clean. Others are constantly opening and shutting their forks, reaching in all directions and grasping and removing anything which may have become entangled in the spines. They also capture floating bits of seaweed, which they drag over the body to conceal it. A third set of projections are the /sphæridia/, small globular bodies said to be connected with perception. [Illustration: Test of sea-urchin, with the spines removed to show the arrangement of the plates, lateral view: /Amb/, ambulacral zone with its perforated plates; /Ap/, apical (aboral) pole; /Int. amb./, interambulacral zones. (From Bronn's "Thierreich.")]