_Ophiopholis_. (_Ophiopholis bellis_ LYM.)
[Illustration: Fig. 148. Ophiopholis, from above; natural size.]
There are but two species of the ordinary forms of Ophiurans in Massachusetts Bay; the white Amphiura (_Amphiura squamata_ Sars), with long slender arms, and the spotted Ophiopholis (Fig. 148), with shorter and stouter arms, and in which the disk is less compact than in the Amphiura, and not so perfectly circular. All Ophiurans are difficult to find, from their exceeding shyness; they hide themselves in the darkest crevices, and though no eye-specks have yet been detected in them, they must have some quick perception of coming danger, for at the gentlest approach they instantly draw away and shelter themselves in their snug retreats. [Illustration: Fig. 149. One arm of Fig. 148; from the mouth side.]
[Illustration: Fig. 150. Ambulacral tentacle of Ophiopholis; magnified.]
They differ from the Star-fishes in having the disk entirely distinct from the arms; that is, the arms, instead of merging gradually into the disk, start at once from its margin. They have no interambulacral spaces or plates; but the whole upper surface is formed of large hard plates, which extend from the back over the sides of the arms to their lower surface, where they form a straight ridge along the centre. (Fig. 149.) The sides of these plates are pierced with holes, through which the tentacles pass; these have not, like those of the Star-fishes and Sea-urchins, a sucker at the extremity, but are covered with little warts or tubercles (Fig. 150); they are their locomotive appendages, and their way of moving is curious; they first extend one of the arms in the direction in which they mean to move, then bring forward two others to meet them, three arms being thus usually in advance, and then they drag the rest of the body on. They move with much more rapidity, and seem more active, than the Star-fishes; probably owing to the greater independence of the arms from the disk. The spines project along the margin of the arms, and not over the whole surface, the back of the arms being perfectly free from any appendages, and presenting only the surface of the plates. The madreporic body is formed by a plate on the lower side of the disk, in a position corresponding to that which it occupies in the young Star-fish; this plate is one of the large circular shields occupying the interambulacral spaces around the mouth. (Fig. 149.) On each side of the arms, where they join the disk, are slits opening into the ovarian pouches. They have no teeth; but the hard ridge at the oral end of the ambulacra, extending toward the mouth in Star-fish, is still more distinct and sharper in the Ophiurans, approaching more nearly the character of teeth.