base of the arms. The Ophiuradæ move themselves by briskly contracting their arms so as to produce a succession of undulations analogous to those by which a serpent creeps along. Some of these zoophytes are rather active; but others attach themselves by their arms to the branches of certain other polyps, like the Gorgons, and remain immovable for a considerable time, waiting their prey somewhat like a spider in the midst of his web. The family of Ophiuradæ is divided into two great sections: that of the Ophiura, which comprehends several genera, amongst others that which gives its name to the family, and that of the Euryalina or Asterophytes. [Illustration: Fig. 110. Ophiocoma Russei (Lutken), natural size.] The family of Ophiuradæ constitute a group distinguished by their five simple, articulated, very mobile, and non-ramified arms, which are attached to a small disk or shield plate, with flexible thread-like cirri between the rays. _Ophiura natta_ is very common, and has been known from very early times in European seas. It is of a greenish colour, with transverse bands, which become more obscure upon the arms as the distance from the disk increases. This disk is from six to seven-eighths of an inch in size, the upper part covered with unequal plates, in shape like tiles; the arms are four times the length of the diameter of the disk, very slender and tapering. The zoophyte to which Lamarck gave the name of _Ophiura fragile_ has now its place among the Ophisthrix, the specific name, indicating a particularity of structure in all these small creatures derived from their fragile formation. In short, these beings have so little consistency that they crumble, as it were, under the touch, and become reduced to pulp under the slightest pressure. In Fig. 110 we give the representation of an Ophiura of the natural size, which Lutken has since called _Ophiocoma Russei_. This Echinoderm, which lives in the seas of the Antilles, is furnished with five very flexible rays, which are armed with from three to four rows of spines, those on the upper part of the body being very hard ones; the body and arms of this creature are of reddish brown, streaked with a great number of little white lines. [Illustration: Fig. 111. Asterophyton verrucosum (Lamarck).]