ASTEROIDEA. 



185 



.T 



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 ; M h huccal 

 membrane ; N circ'umoral nerve- 

 ring ; T Tiedemann's body ; 1!' 

 water- vascular ring ; Zoral portion 

 of deep oral nervous system 

 (Lange's nerve). 



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 

 and 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 

 i n t e r r a dius ; in 

 the latter case it 

 is common to find 

 several vesicles 

 opening by one 

 stalk. It is sup- 

 posed by some 

 that the polian 

 vesicles are con- 

 tractile structures 

 acting as central 

 organs to vary the 

 pressure in the 



10 



FIG. 136. Circumoral water-vascular vessel with appendages of 

 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 poliau vesicles ; 7 Tiede- 



A , r l ir .1 c 

 n 



T TQ( 



C , 7a f m 



fe j bl [11 > 



mann's bodies ; 8 circumoral vessel ; 9 ring of supposed vascular fi, c fV, f fl,,, 

 tissue; 10 ampullae of tube-feet. Otlieib tnat tliey 



