IX 



PHYLUM ECHINODERMATA 



411 



The tube-feet are arranged in a double row along each of the 

 ambulacral grooves, each connected through an aperture between 

 the ambulacral ossicles with an ampulla, or, exceptionally, with two 

 ampullae, situated in the coelome. Each double row of tube-feet 

 terminates at the extremity of the arm in an unpaired appendage, 

 the tentacle, which is tactile and olfactory, and not locomotive in 

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

 terminal suckers. 



In the Ophiuroidea (Fig. 341) the central disc is much more 

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

 arms, which are usually five in number, rarely six, are comparatively 

 slender and cylin- ^ 



drical, tapering to- 

 wards the free ex- 

 tremities ; in one 

 group, the Euryalida 

 (Fig. 342), they are 

 branched. The 

 mouth is in the 

 middle of the oral 

 surface of the disc, 

 as in the Asteroidea, 

 but there are no 

 ambulacral grooves, 

 and there is no anal 

 aperture. Five pairs 

 of slits on the oral 

 surface (Fig. 341, C) 

 lead into the genital 

 bursse, which receive 

 the sperms and ova 

 from the gonads, and 



which appear also to 

 act as Organs OI 

 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 

 of fin-like appendages, supported by slender spines, on each joint 

 of the arms. The madreporite is situated inter-radially on the 



. 341. 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.) 



