

FIG. 135. Vertical section through 

 an interradial region of the peri- 

 stom of Asterias rubens L. (after 

 Ludwig), showing the connexion 

 of one of Tiedemann's bodies with 

 the water- vascular ring. B tissue 

 of the so-called vascular ring ; 

 E outer perihaemal ring ; J inner 

 perihaemal ring ; Mi interradial 

 muscle of peristom ; Mh buccal 

 membrane ; N circumoral nerve- 

 ring ; T Tiedemann's body ; W 

 water- vascular ring ; Z oral portion 

 of deep oral nervous system 

 (Lange's nerve). 



ASTEROIDEA. 185 



Cubical ciliated yellow epithelium. 



There are usually two in each inter- 



radius, except that of the madre- 



porite, which has only one, and they 



are supposed to be of the nature of 



lymphatic organs and to bud off 



cells into the water- vascular sys- 



tem. 



Like so many other features of 



Asteroid anatomy the polian vesicles 



vary considerably both in number 



a,nd arrangement throughout the 



class, even in closely allied forms. 



They are large vesicular structures 



with muscular walls and long stalks 



which open interradially into the 



circumoral vessel (Fig. 136). Some- 



times (Asterias rubens and glacialis, 



etc.) they are absent altogether ; 



sometimes there is one in each interradius, except that 



of the madreporite, in which there are none at all or 



two (species of Astropecten) ; finally there are cases of two, 



three or four or 

 even more in each 

 interradius; in 

 the latter case it 

 is common to find 

 several vesicles 

 opening by one 

 It is sup- 

 by some 

 that the polian 

 vesicles are con- 

 tractile structures 

 acting as central 

 organs to vary the 



FIG. 136. Circumoral water-vascular vessel with appendages of Pressure in the 

 Asterina gibbosa (after Cuenot, from Lang), seen from the aboral 

 side. 1 mouth in the centre of the buccal membrane ; 2 stone- 

 canal; 3 axial sinus ; 4 transverse muscles of the ambulacral 

 ossicles; 5 ambulacral ossicles ; 6 polian vesicles ; 7 Tiede- 

 mann's bodies ; 8 circumoral vessel ; 9 ring of supposed vascular 

 tissue ; 10 ampullae of tube-feet. 



stalk. 



wo for tra a 



eirafom Vv 

 te J &l 1U ' U J 



rfViore fViaf 

 OtrierS tnat 



