. The Cambridge natural history. Zoology. 512 ECHINODERMATA ECHINOIDEA 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 droebachiensis four pairs. The ambulaeral plate is really made up of a series of " pore-plates," -each carrying a single pai
. The Cambridge natural history. Zoology. 512 ECHINODERMATA ECHINOIDEA 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 droebachiensis four pairs. The ambulaeral 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 primi- FiQ. 228.—The so-called calyx and the peri- proot of Echinus esculentus. x 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 irregu- lar plates; 5, anus. (After Chadwick.) tive 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble t
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