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MANUAL OF ZOOLOGY 



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



— which are placed inter-radially. On the sides of the neck 

 of each Polian 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 vesicles, or Tiedemann' s vesicles, the interior of 

 each of which is divided into a number of chambers. 



Fig. 90. — Ambulacral system of a starfish, a, ampullae; ap, Polian vesicles; c, 

 circular canal; m, madreporite; m' , madreporic canal; p, tube-feet; r, radial 

 vessels; r' , branches to ampulla:. (After Gegenbaur.) 



Accompanying the madreporic canal there is an organ, — 

 the ovoid gland, — the relationships and function of which 

 have given rise to a considerable amount of difference of 

 opinion. It is a fusiform body, the interior of which is 

 divided up into a number of freely communicating spaces. 



