Fig. 139. Sucker-plate of Sea-Egg.
Fig. 140. Section of a sucker-plate.
The mechanism for extending and retracting the feet by a liquid, is the same with that in the star-fishes, but the pores which admit the liquid into the feet are double. The tubular feet swell at their extremity into a fleshy sucker, within which there is a thin glassy reticulated rosette ([fig. 139]), of which [fig. 140] is a highly magnified segment. It is perforated in the centre by a large round opening. The sea-urchins can stretch their feet beyond the spines, and by means of the suckers they can attach themselves even to smooth objects, or aided and directed by their spines they roll themselves along with a rotatory motion head downwards.
The circulation of the bright yellow blood is like that of the star-fishes. It is aërated both internally and externally. The external respiratory organs are short, branched, and highly vibratile bodies attached in pairs to the oval extremities of the fine imperforated bands.
There are pedicellariæ scattered among the spines of the sea-urchins which are in constant motion, protruding themselves beyond the spines and withdrawing again, snapping their pincers, and grasping firmly anything that comes within their reach, or that is presented to them. The pedicellariæ vary much in form and position in the different genera of the Echinidæ; but they invariably consist of a long, slender, calcareous stem, and generally tripartite head, the whole coated with a gelatinous fibrous transparent substance. The head of the Pedicellaria globosa is a formidable weapon; at the apex of each of its three serrated and toothed blades there is a strong sharp spine directed horizontally inwards, so that the three spines cross each other when the blades close, which they do so energetically that nothing could escape from such a grasp. The pedicellariæ are curious microscopic objects; they are extremely irritable, and although their use is unknown, they must be essential to the well-being of the animals, since hundreds are scattered over their shells.
The spines of the Echinidæ vary in shape and structure in the different genera and species. Those of the Scutella form merely a velvety pile. On the common sea urchin the spines are simple, and shed twice in the year; those on the Amphidetus are both club and spoon-shaped; and, on the Cidaris, they are large formidable clubs moved by a ball and socket. All the spines, whatever their form may be, are moved in that manner; for there are little tubercules on the surface of the shell on which a cup at the bottom of the spines is pressed down by the muscular skin which covers the shell and spines, and by its contractile power it enables the animal to move the spines in any direction.
The microscopic structure of the calcareous spines is often beautifully symmetrical. Those of the Acrocladia mamillata consist of concentric alternate layers of network and sheaths of pillars; so that a section of the spine perpendicular to its axis exhibits a succession of concentric rings like those of an exogenous tree. The cup at the bottom of the spine is very dense network, and the last of a sheath of encircling pillars form the ribs, sometimes seen on the exterior of the spines.
