The /Echinoidea/ present great differences in shape, being more or less spherical, oval, discoid, and heart-shaped. These variations are associated with the differences of internal structure, the openings of the digestive tract being at the opposite poles in the spherical and oval forms, but excentric in the disk- and heart-shaped species. The sea-urchins are grouped in three orders in accordance with these variations. All are characterized by the absence of arms, by having the calcareous plates immovably united to form a firm test, and by the great development of the movable spines upon the plates. Sea-urchins are sometimes called /sea-eggs/, perhaps from their shape, but possibly from the edible quality of some species, which are eaten by the natives of the shore, who take them at the spawning season, when the egg-sacs are distended. They are gregarious, and frequently are so crowded together as literally to pave the surface of rocks and the bottoms of tide-pools in sheltered places. The following is quoted from A. Agassiz: "Many of the /Desmosticha/ along coasts exposed to the action of the waves live in cavities which they hollow out of the solid rock. This they do, not by means of any solvent, but by mere mechanical action. They chisel out with their teeth the solid rock by incessant turning round and round, and keep their cave, where they are frequently prisoners for the rest of their existence, up to the size required by the growth of their test and spines, by constant gnawing. On the coast of California the common /Strongylocentrotus purpuratus/ occurs in this way. We find long tracts of shore, where this sea-urchin is common, completely honeycombed and pitted by cavities and depressions in which they seek shelter against the powerful surf continually beating against the rocks. The same species does not excavate in sheltered places, where the sea-urchins can find protection between the interstices of large fragments of rock or ledges more or less sheltered from the more direct action of the open sea." Sea-urchins in cavities of granite rock, where the openings are too small for the animal to get out, are to be seen in thousands on the coast of France at Croisic, Lower Loire. Spines of large sea-urchins are used as slate-pencils by the missionaries in the Pacific Islands. [pg222] FAMILY ~CIDARIDÆ~ GENUS ^Cidaris^ ^C. tribuloides.^ Similar to /Porocidaris sharreri/, but with thicker and stouter spines. Found from South Carolina to Brazil. GENUS ^Dorocidaris^ ^D. papillota.^ A deep-water species which occurs off Chesapeake Bay and southward. It has slender spines with distinct longitudinal rows of serrations, and the spines are grouped in rosette-like forms over the small spherical body. ^D. Blakei.^ This species is very peculiar in having broad, fan-shaped spines; vermilion in color. Found in deep water in the Bahamas and West Indies. (Plate LVII.) GENUS ^Porocidaris^