386 



ZOOLOGY 



SECT. 



take the special form to which the term paxillcc is applied. Each 

 paxilla is a plate which is produced into a short rod, divided at its 

 extremity into a number of radiating processes. 



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, 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 



. 30(i. OphiOglypha lacertosa. A, outline, of the natural size. , central disc, dc 

 view. C, the disc, ventral view showing the mouth and genital fissures. (From Nichols 

 and Lydekker's Paleontology.') 



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

 terminal suckers. 



In the Ophiuroidea (Fig. 306) 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, 

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

 the Eitrt/ft/idd (Fig. 307), they are branched. The mouth is in th 

 middle of the ventral surface of the disc, as in the Asteroidea, bu1 

 there are no ambulacral grooves and there is no anal aperture. 

 Five pairs of slits on the oral surface (Fig. 306, C) lead into the 

 genital sacs, 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 



