IX 



PHYLUM ECHINODERMATA 



377 



wide, five-sided, ring-like canal, the ring-vessel of the ambulacral 

 system. From this are given off the five radial ambulacral vessels, 

 passing to the extremities of the arms. From the pentagonal canal 

 are given off also in most Starfishes, but not in Asterias, a series 

 of five pairs of appendages, the Polian vesicles (Fig. 313, pol. ves) 

 pear-shaped, thin-walled bladders with long narrow necks which 

 are placed inter-radially. At the sides of the neck of each Polian 



FIG. 314. A, view of the under part of a specimen of Asterias rubens, which has been 

 horizontally divided into two nearly equal portions. 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 or axial organ ; marg. 

 marginal ossicle ; nerv. circ. nerve-ring ; oe. cut end of oasophagus ; pst. peristome ; ret. 

 retractor muscle of the stomach ; sept, inter -radial septum ; stone, c. stone-canal ; T. 

 Tiedemann's vesicle ; w. v. r. \vater-vascular ring-canal. (After MacBride.) 



vesicle (except in the inter-radius containing the madreporic canal, 

 where there is one on one side only) project inwards a pair of 

 little rounded glandular bodies, the racemose or Tiedemann's vesicles 

 (Fig. 314, T), the cavity in the interior of each of which, opening 

 into the ring-vessel, is divided into a number of chambers. 



The various parts of the ambulacral system of vessels have a 

 muscular wall and an internal lining epithelium in addition to the 



