ASTEROIDEA 



637 



cavity between the arms. To the septum in the interradius of the 

 madreporite is attached a sac, the axial sinus, and into this, so as to 

 appear to lie in it, project the axial organ and the stone canal, whose 

 wall is calcified and infolded so as to increase its surface. Orally, the 

 stone canal joins the water vascular ring, which bears nine small 



Fig. 443. Part of the aboral half of a starfish (Asterias ruhens) removed, with 

 the alimentary canal, from the rest of the body, and viewed from within. One 

 lobe of the stomach has been cut away, and another partly turned back. The 

 detached figure represents an enlarged view of the axial sinus and adjoining 

 structures. From Borradaile. abo.m. aboral muscle; ax.o. axial organ; ax.s. 

 axial sinus ; l.st. one of the lobes of the stomach ; oes. oesophagus ; py.c. pyloric 

 caecum ; py.d. pyloric duct ; py.s. pyloric sac ; r.an. rectal caecum ; sep. septum ; 

 st.c. stone canal. 



TiedemanrCs bodies, of gland-like structure, and often, but not in 

 Asterias, several stalked sacs, the Polian vesicles. The radial water 

 vessel of each arm lies under the ambulacral ossicles, and between 

 them and the radial nerve is the perihaemal vessel, divided by a 

 septum in which runs the "blood vessel". The gonads are ten in 



