All parts of this creature, which is called a Pluteus, are strengthened by a framework of calcareous rods tipped with orange colour, all the rest being transparent and colourless. It swims freely, back foremost, by means of its cilia.
[Illustration: Fig. 143. Larvæ of Echinus in various stages of development within the Pluteus, which is not represented:—B, disk with the first indication of the cirrhi; C, disk with the origin of the spines between the cirrhi; D, more advanced disk with the cirrhi, _g_, and spines, _x_, projecting from the surface.]
While in this active state a circular disk (_c_, fig. 142), covering the stomach (_b_, fig. 142), appears within it, which gradually expands, and sends through the skin of the Pluteus spines, pedicellariæ, and tubercules, ultimately developed into hollow feet. Then the feet are pushed out and drawn in, the pedicellariæ (D, fig. 143) snap their pincers; and while the half-formed Echinus is making these motions within the Pluteus, the mouth and gullet of the Pluteus itself are in constant activity; and, while it swims about, the unformed Echinus within it gets a globular shape, the shell is formed, and when the Echinus is complete, the rest of the Pluteus is thrown off, and the young animal rolls away.
The free swimming larval zooids of the Echinodermata are generally hyaline, and some are phosphorescent. The Pluteus is also the larval zooid of the ophiurid star-fishes; they may be seen in great numbers on the surface of the sea in August and September. The young star-fish is formed in them by a process analogous to that described. The motions of the Echinidæ are reflex; nothing indicates volition.
The fossil Echinidæ first appeared in the lower Ludlow limestone, and attained their maximum in the Cretaceous strata. A species of Diadema, with annulated hollow spines, common in the Chalk, still exists. Numerous species of the genus Clypeaster, remarkable for their flattened form, and known as lake urchins, are peculiar to the Tertiary strata and existing seas; and, lastly, five species of Spatangidæ, heart-shaped urchins, which lived in the Tertiary periods, still exist. In consequence of the porous texture of the solid calcareous parts of the Echinidæ, their fossil remains are commonly impregnated with pyrites or silex, without altering their organic structure, so that they exhibit a fracture like that of calcareous spar.