1911 Encyclopædia Britannica/Sea-Urchin

SEA-URCHIN. These animals belong to the great group of Echinoderms (see Echinoderma) and to its class Echinoidea. Both the scientific and the English names denote their resemblance to the urchin or hedgehog, the resemblance lying in the prickles with which the skin is covered. The skin itself is stiffened by a deposit of Calcite (crystalline carbonate of lime) in the form of plates. If the prickles be scraped away, these plates will be seen to form a hard shell or test, in which are two openings, for the mouth and the anus. According to the position of these openings the urchins are described as Regular or Irregular. In the Regular urchins, of which Echinus esculentus, the edible egg-urchin (fig. 1), and Dorocidaris papillata, the piper (fig. 2), are familiar examples, the test is spheroidal with the mouth at the lower pole and the anus at the upper. In the Irregular urchins, of which Spatangus purpureus, the purple heart-urchin (fig. 3), is a common type, the test has been drawn out into an oval or heart shape, with the mouth shifted towards the front end and the anus towards the hinder end.

Fig. 1.—A Regular Sea-urchin, Echinus esculentus. The test is still covered with spines, the suckers of the podia are seen in ten rows.

The greater part of the test of a Regular urchin is divided, as a globe by meridians of longitude, into ten areas, each composed of two columns of plates. In five of these areas the plates are pierced by pairs of pores (fig. 2, Ambulacrum), and in life there issues from each pair a tubular process with a sucking disk at its end (fig. 1). Within the test these processes or podia are connected with five tubes arising from a tubular ring round the mouth and running upwards to the apex, where each passes out as a single process through a special plate at the end of the area to which it belongs. Since this terminal process is sometimes surrounded by pigment, as are organs susceptible to light, it has been regarded as an eye and the plate through which it passes called an ocular (fig. 2). From the ring-canal round the mouth a single tube passes straight through the body-cavity to the apex, where it opens through a sieve-like plate—the madrepore (g. 2). Thus all this system of tubes is placed in connexion with the outer sea-water, and is filled with it. Within the test the bottom of each podium is swollen into a little bag—ampulla—likewise full of water, and when the muscles with which it is provided pull the sides of the bag together, the water is squeezed into the podium and dilates it, so that it is stretched far out (see Echinoderma, fig. 12 D). The podium can then wave about and attach its sucker to any smooth object within reach. Each of these five areas, with the podia on each side of it extended and waving, looks like a garden avenue—Latin ambulacrum—and the areas are therefore called ambulacral areas, the plates composing them ambulacrals, and the whole system of water-vessels the ambulacral system. This system forms perhaps the most characteristic feature of all living Echinoderms, but it reaches its highest development in the urchins. The five areas alternating with tire ambulacral areas are called interambulacral (fig. 2, Interambulacrum); their plates are not pierced by pores but are generally ornamented by large tubercles bearing big prickles (spines or radioles), between and around which are smaller prickles (fig. 2). The madreporite is one of five plates that surround the anal opening and alternate in position with the oculars. Each of these plates is pierced by a pore, connected on the inside with one of the five generative glands, and giving passage to the eggs or milt when they are ripe; hence these plates are called genitals (fig. 2). The five genitals and five oculars together form the apical system of plates (see Echinoderma, fig. 3, A.B.). From the mouth to the anus the gut follows a coiled course, first going round the cavity of the test in one direction and then turning back on itself, while the two limbs of the loop thus formed are themselves thrown into festoons attached by strands to the wall of the test. The lower coil, next the mouth, is the stomach in which food accumulates, while the upper coil is the intestine roper. In Echinus, but not in the Cidarids, a narrow tube branches from the gut at the beginning of the first coil, runs alongside the stomach, and re-enters the gut at the end of the coil; this, which is called the siphon, permits a flow of water through the gut however full of food the stomach may be. Round the gullet is a jaw-apparatus, consisting essentially of five hard, pointed teeth, the ten jaw pieces in which they are held, five struts between the pairs of jaws, and five cambered stays for the attachment of ligaments to keep the whole apparatus in position. The jaws are worked by muscles in such a way as to draw the teeth together or apart, inwards or outwards. This apparatus is often called “Aristotle’s lantern,” though it is extremely doubtful whether Aristotle (Hist. Anim. iv. 5) was alluding to this structure. The whole of it is covered by the membrane lining the body-cavity, and from the space thus enclosed there pass to the exterior five pairs of hollow branched appendages, the external gills; the five notches through which the gills passed can be seen in the dried test of an Echinus from which the mouth membrane has been removed, but not in the test of the piper-urchin or other Cidarid, because there the gills are not developed.

Fig. 2.—A Regular Sea-urchin, Dorocidaris papillata. The test seen
from above, with most of the spines removed. Natural size.

The prickles that cover the test are better studied in the piper urchin (fig. 2), where some of them are very large and, from their resemblance to the drones of a bagpipe, have suggested the name of the animal. Each of these large spines or radioles is attached to a rounded tubercle b an enclosing ligament and outer coat of muscles, the base of the racliole being hollowed to fit on the tubercle. Thus the radiole can be moved in any direction. The attachment of the larger radioles is protected by a ring of smaller ones. These and the other small spines protect the sea-urchin, as its prickles protect a hedgehog; the larger ones may also help the animal to move or to fix itself firmly against the shock of waves. Some urchins, especially the purple egg-urchin, bore holes even in very hard rocks, and by stretching out their radioles they can hold themselves immovably in their holes; how they bore the holes is not known with certainty. Besides radioles, small pincer-like appendages called pedicellariae are attached to the test by similar ball-and-socket joints. Each consists of a long stalk bearing three blades which can meet at their points; on the inner surface of each blade is a cushion of sensitive skin, and often a gland which secretes a poison. The pedicellariae were once supposed to be parasites, but they are really organs of the urchin of the same nature as the radioles; they are of four different
Fig. 3.—An Irregular Sea-urchin,
Spatangus purpureus.
forms, three of which undoubtedly serve for defence, while the shortest ones clean the test from impurities and sand-grains that fall between the radioles. Sea-urchins other than Cidarids also bear on the test minute sensory organs called sphaeridia, each consisting of a small hard knob, supported by a stalk which may be partly calcified but always contains many nerve-fibres. It is generally supposed that they are sensitive to vibrations in the water, and to any change from the normal position which the animal may assume or be forced into. Such a regular urchin as has here been described lives with the mouth downwards, preferring a hard floor, on which it creeps by its podia and its radioles, constantly scraping the algae and seaweeds from the rock with its teeth and so feeding itself. If it does not bore a hole, or is not protected by long needle like radioles, it may grasp bits of sea-weed or other objects with its pedicellariae and hide beneath them from the fish that seek it for food.

The Irregular urchins (fig. 3) have been modified for another way of life. Some of them live in mud or ooze, through which they creep. The mouth has moved forward, has lost its jaws and often has a lip, projecting so as to scoop up the mud. The prickles have become smaller, often almost silky, and are generally directed backwards so as not to oppose the passage of the body. The podia of the under surface still aid locomotion, but those of the upper surface, which are concentrated in five petal-shaped areas, act mainly as gills. These urchins often assume a heart shape, owing to the greater development and sinking in of the front petal. The sand dollars and their allies, which live half-buried in sand without moving through it, retain a more or less circular outline, as well as the central position of the mouth, which has not lost its jaws; the anus, however, has moved to the side, while the podia of the upper surface are concentrated in petals and many of them modified into branched gills. The sand-dollars proper are very thin and flat, but the shield-urchins (Clypeaster, &c.) have the central region of the upper surface raised in a boss, which reaches above the sand, so that the animal can still breathe though the whole body is hidden. In many Irregular urchins the petals of the ambulacra are deeply sunk, and serve as a nursery for the young, which are covered by the spines of the parent.

Sea-urchins live only in the sea, from between tide-marks down to all but the greatest depths. The abyssal forms have very thin tests, which are often flexible. Urchins eat all kinds of animal and vegetable food, and are themselves attacked by fish, by star-fish, and even by other urchins. The ripe egg-bunches area favourite article of diet with dwellers round the Mediterranean; in other respects sea-urchins are of small importance to man, being neither useful nor harmful. In olden times the larger radioles were recommended to be powdered and taken as a remedy for the stone.

For details of classification, see under Echinoidea, in the article Echinoderma.