Fig. 226.—Views of a single blade of each kind of pedicellaria. A, blade of gemmiform pedicellaria of Echinus elegans; g, groove for duct of poison gland; B, blade of tridactyle pedicellaria of the same species; C, blade of ophicephalous pedicellaria of the same species; r, ring for clamping this blade to the other blades; D, blade of trifoliate pedicellaria of E. alexandri. (After Mortensen.)
The ophicephalous pedicellariae, with their powerful bull-dog grip, assist in holding small animals, such as Crustacea, till the tube-feet can reach them and convey them to the mouth.
The number and variety of the pedicellariae, then, is an eloquent testimony to the dangers to which the soft sensitive skins of the Sea-urchin and other Echinodermata are exposed, and afford confirmatory evidence in support of the view expressed above, that the method adopted to defend the skin was one of the great determining features which led to the division of the Asteroidea into different races.
Fig. 227.—Dried shell of Echinus esculentus, showing the arrangement of the plates of the corona. × 1. 1, The anus; 2, periproct, with irregular plates; 3, the madreporite; 4, one of the other genital plates; 5, an ocular plate; 6, an interambulacral plate; 7, an ambulacral plate; 8, pores for protrusion of the tube-feet; 9, tubercles of the primary spines, i.e. primary tubercles.
The corona consists of five radial or "ambulacral" bands of plates and five interradial, or as they are usually termed, "interambulacral" bands of plates—ten in all. Each of the ten consists of two vertical rows of plates throughout most of its extent, and each plate is studded with large bosses, or "primary tubercles" for the primary spines, smaller bosses called "secondary tubercles" for the secondary spines, and finally, minute elevations called "miliary tubercles" for the pedicellariae.
Fig. 228.—The so-called calyx and the periproct of Echinus esculentus. × 4. 1, Genital plates with genital pores; 2, ocular plates with pores for terminal tentacles of the radial water-vascular canals; 3, madreporite; 4, periproct with irregular plates; 5, anus. (After Chadwick.)
Even in the dried skeleton, however, the ambulacral plates can be discriminated from the interambulacral by the presence of pores to permit the passage of the tube-feet. These pores are arranged in pairs, and each pair corresponds to a single tube-foot, since the canal connecting the ampulla with the external portion of the tube-foot is double in the Echinoidea. In Echinus esculentus there are three pairs of such pores in each plate, in Strongylocentrotus droëbachiensis four pairs. The ambulacral plate is really made up of a series of "pore-plates," each carrying a single pair of pores, and these become united in threes in Echinus and fours in Strongylocentrotus, while in primitive forms like the Cidaridae they remain separate. Each ambulacral and interambulacral area ends at the edge of the periproct with a single plate. The plate terminating the ambulacral band is pierced by a single pore for the exit of the median tentacle, which, as in Asteroids, terminates the radial water-vascular canal. Thus the aboral end of the radius in an Echinoid corresponds to the tip of the arm in an Asteroid. The plate is termed "ocular," because the terminal tentacle has a mass of pigmented cells at its base; but no eye-cups can be seen, and there is no evidence that this spot is specially sensitive to light. Species which show special sensitiveness to light have often a large number of what we may perhaps term secondary eyes. The plate terminating the interambulacral series is termed the "genital plate," because it is pierced by the duct of one of the five genital organs. One of the genital plates is also pierced by the madreporic pores. Some zoologists have separated the ocular and the genital plates under the name of "calyx" from the rest of the corona, under a mistaken idea that they are homologous with the plates of the body or calyx of a Crinoid.