420 



ZOOLOGY 



function. ,The tube-feet are provided (except in Astropecten) with 

 terminal suckers. 



In the Ophiuroidea (Fig. 336) the central disc is much more 

 sharply marked off from the arms than in the Asteroidea. The 

 arms, which are five in number, are comparatively slender, and 

 cylindrical, tapering towards the free extremities ; -in one group, 

 the Ewrycdida (Fig. 337), they are branched. The mouth is in the 

 middle of the oral surface of the disc, as in the Asteroidea, but 

 there are no ambulacra] grooves, and there is no anal aperture. 



Fig. 336.— Ophioglypha lacertosa. A, outline, of the natural size. B, central disc, aboral 

 surface. C, the disc, oral surface showing the mouth and genital fissures. (From Nicholson 

 and Lydekker's Paleontology.) 



Five pairs of slits on the oral surface (Fig. 336, C) lead into the 

 genital bursas, which receive the sperms and ova from the gonads, 

 and which appear also to act as organs of respiration and perhaps 

 also of excretion. The surface is covered with thin plate-like 

 ossicles, usually beset along their edges with longer or shorter 

 spines ; sometimes irregular calcareous granules take the place 

 of plates. Hook-like organs of adhesion are present only in the 

 Euryalida. Each of the arms is supported by a row of internally 

 situated ambulacral ossicles. Tube-feet are present and are pro- 

 truded at the sides of the arms between the lateral plate-like 

 ossicles; but they have no sucking-discs and no ampullae, and 

 locomotion is effected in the majority of the Ophiuroids by active 

 flexions and extensions of the arms. In one genus there is a pair 



