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erable size, attached externally to articulate with the moveable locomotive spines. Each of these large plates has somewhat of a pentagonal form, those situated near the mouth and apex being considerably smaller than those at the centre of the shell;—there are ten rows of these tuberculated plates, but being disposed in pairs, each row of large pieces being united by a zig-zag suture with another of a similar description, there are in reality but five large segments of the shell, each supporting a double row of tubercles. Nor are these the only parts to which spines are affixed, a vast number, though of secondary importance, being disseminated over the surface. In addition to the plates supporting the tubercles, there are a number of ambulacral bands which support the myriads of suckers protruded after the manner of the Starfishes. "In a moderate sized Urchin," Forbes tells us,[1] "he reckoned sixty-two rows of pores in each of the ten avenues." Now as there are three pairs of pores in each row, their number multiplied by six, and again by ten, would give the great number of 3,720 pores; but as each sucker occupies a pair of pores, the number of suckers would be only half that amount, or 1,860. In other parts its structure is not less complicated,—there are above three hundred plates of one kind, and nearly as many of another, all dovetailing together with the greatest nicety and regularity, bearing on their surfaces above 4,000 spines, each perfect in itself, of a complicated structure, and having a free movement in its socket,[2]
