any animal belonging to the Echinoidea, a class of Echinoderma (q.v.). The simplest living forms are the Cidaridae (Cidaris, "jewelled tiara"). In them the test when de nuded of prickles, which thickly cover its surface, is of flattened turban shape, beautifully ornamented with tubercles. The test is built of thin but solid plates closely united. It is divided into five parts by five double columns of smaller plates, and to each of these plates is attached an extensile tube (podium, or "little foot") connected through two holes in the plate with the hydraulic sys tem inside the test. These double columns are called ambulacra ("garden walks"). The five portions of the test between them are the interambulacra, and each is composed of larger plates, also arranged in double alternating columns. The terminal plate at the upper end of each ambulacrum is pierced with a pore for a sensitive papilla, and is termed an "ocular," because in sea-stars the corresponding plate bears an eye. Between the oculars and at the top of the interambulacra are five larger plates, each pierced with a pore for the extrusion of the generative products ; they are therefore termed "genitals." One of the genital plates has also many small pores leading to the hydraulic system, and is termed the "madreporite." Within the circlet formed by these ten plates is a finely-plated membrane through which opens the vent. On the under side of the test is a larger circular space covered with membrane, in the centre of which is the mouth. The pore-plates of the ambulacra pass over the membrane to the edge of the mouth.
Each interambulacral plate bears a large tubercle, set in a circlet of smaller tubercles. Each main tubercle bears a long prickle or "radiole," attached to it by a socket and a covering of muscles, which can move the radiole in any direction. The joint is pro tected by flattened prickles or radioles attached to the smaller tubercles. Prickles also protect the various pores and the delicate organs issuing from them. This species is dredged at depths of 5o to 1,80o metres along the east coast of the Atlantic, and Scottish fishermen call it the piper, from the resemblance of its long radioles to the drones of a bagpipe.
Projecting from the mouth of a Cidaris are five pointed teeth; these are long, curved and chisel-ended, and are supported by a frame-work of 3o bones, with the necessary muscles, forming a five-sided structure which Aristotle compared to a ship's lantern. From the top of the lantern rises the gut, which passes in a looped curve round the inside of the test till it reaches the vent. Inside the test are also the five double rows of hollow balls con nected with the podia (see ECHINODERMA, fig. 7), and in the interspaces the five genital glands. The suckers of the podia are poorly developed, and are chiefly used as feelers when the animal walks about in search of food; because they have no gripping power, Cidaris is not found in waters disturbed by waves and currents. Cidaris usually moves by using the large radioles as stilts. It can climb over obstacles, and has even been seen to swarm up a glass rod by grasping it with the curved and serrate radioles round the mouth; it uses these also to seize its prey.
Though the small prickles protect the openings in the test, the true defensive organs are its pedicellariae, scattered over the test between the prickles. These are like little pincers, each having
three teeth at the end of a flexible stalk, and each tooth with a poison-gland. Pedicellariae are found in all sea-urchins, but differ in size and shape according to the genus. Besides gripping and poisoning enemies and prey, pedicellariae clean the test from particles of dirt, and in some shallow-water urchins they hold bits of sea-weed on the upper surface of the test to hide it.
The most familiar British echinoid is the egg-urchin, Echinus esculentus (ECHINODERMA, fig. 3), which owes its name to the fact that the ripe ovaries are eaten, both raw and cooked. Essen tially a shore-dweller, it grips the sea-weed with its podia, which have well-developed and are far more numerous than in Cidaris; E. Forbes estimated their number as 1,860. To accom modate them, the number of plates in the test is increased, and the ambulacral plates are crowded, so that the pore-pairs are arranged in oblique groups of three on each side of the avenue, instead of in single rows as in Cidaris. The prickles are all short, and locomotion is effected mainly by the podia; the animal can also drag itself along by its teeth.
The purple egg-urchin (Paracentrotus lividus), which ranges from the south of the British Isles to the Azores and into the Mediterranean, lives among Zostera leaves and on rocky shores in holes which it bores in the rocks. Some of the turban-urchins (Diadema), which live on rocky bottoms in smoother water, have very long thin prickles, which can pierce the stoutest boot ; some of these urchins have organs sensitive to light and shade, so that they quickly direct their prickles to the point of danger. Reef dwelling urchins exposed to the surf have their prickles thickened and crowded, and sometimes the ends are expanded like parasols to form a complete outer covering.
In some deep sea-urchins, as in most of those that lived in early ages, the test is flexible, and in one of them the podia are protected by sharp prickles each bearing a poison-gland. These and all previously mentioned are of regular five-rayed symmetry. Many urchins that have taken to live in sand or mud are irregular. In them the mouth tends to move forward, and to become wide, with projecting under-lip ; the vent moves from the top to the hinder end of the test ; the prickles are fine and directed back wards ; the podia of the upper surface subserve respiration and are concentrated in five petal-shaped areas. Among such forms are the shield-stars or cake-urchins (Clypeaster), which live just below the surface of the sand, and use their reduced teeth for shovelling sand, with its small animals, into the mouth. The sand dollars (Scutella) are thin and flat, and in some the test grows out like the spokes of a wheel (Rotula). More familiar are the heart-urchins, such as the common Echinocardium cordatum (ECHINODERMA, fig. 19), which lives at the bottom of a burrow and has the podia of its front ambulacrum modified so that they can be stretched right up the tube of the burrow to collect food.
The position of sea-urchins in the Economy of Nature has been mentioned in ECHINODERMA (q.v.).
BIBLIOGRAPHY.-Besides works mentioned under ECHINODERMA, a specialist work is H. L. Clark, Catalogue of the Recent Sea-urchins in the British Museum (1925) ; an illustrated popular account is in Ani mals of all Countries (1924). (F. A. B.)