Fig. 141. Spine of Echinus miliaris.
The spines of the Echinus miliaris, of which [fig. 141] represents the segment of a section highly magnified, are fluted columns of calcareous glass, the grooves of which are filled with solid glassy matter curved on the exterior. The innumerable hair-like objects attached to the shells of some of the Echinidæ, the almost filamental spines of others, and the pedicellariæ themselves, are formed of a regularly reticulated substance. When the Echinidæ are stripped of their spines and all their appendages, their shells show 2,400 plates united with the symmetry of a tesselated pavement.
Fig. 142. Pluteus of the Echinus:—a, mouth; b, stomach; c, echinoid disk; d d d d, four arms of the pluteus-body; e, calcareous framework; f, ciliated lobes; g g g g, ciliated processes of the proboscis.
The Echinidæ are male and female, and the eggs are excluded through the five perforated productive plates at the posterior end of the shell. According to the observations of Prof. Fritz Müller the embryo, soon after issuing from the egg, takes a form represented (magnified) in [fig. 142].
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.
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.