442 



ECHINODERMATA ASTEROIDEA 



the ambulacral groove, and a little below this spot a short 

 transverse canal connects the tube-foot with the radial canal 

 which lies beneath these ossicles (Fig. 191). 



The tube-feet are, therefore, really a double row of lateral 



190. A, view of the under half of a specimen of Asterias rubens, which has been 

 horizontally divided into two halves. B, enlarged view of the axial sinus, stone- 

 canal and genital stolon cut across. amb.oss, Ambulacral ossicle ; amp, 

 ampullae of the tube-feet ; ax.s, axial sinus ; gon, gonad ; g.stol, genital stolon ; 

 marg, marginal ossicle ; nerv.circ, nerve ring ; oe, cut end of oesophagus ; pst, peri- 

 stome ; ret, retractor muscle of the stomach ; sept, interradial septum ; stone c, 

 stone-canal ; T, Tiedemann's body ; w.v.r, water- vascular ring-canal. 



branches of the radial canal. The appearance of being arranged 

 in four rows is due to the fact that the transverse canals connect- 

 ing them with the radial canal are alternately longer and shorter 

 so as to give room for more tube-feet in a given length of the 

 arm. Each tube-foot ends in a round disc with a slightly 

 thickened edge. The radial canal terminates in a finger-shapec 



