560 THE INVERTEBRATA 



The mouth leads through a short oesophagus (Fig. 416) into a 

 large sac-like stomachy with two retractor muscles in each arm. Above 

 is a five-sided pyloric sac, from each angle of which, separately or, as 

 in Asterias, by a short common duct, arises a pair of branched pyloric 

 caeca^ which are slung, each by a double mesentery, from the roof 

 of an arm. From the pyloric sac a short, conical rectum, bearing in 

 Asterias two glandular rectal caeca, rises to the anus, which is slightly 

 excentric, in the interradius which is next, clockwise, after that of the 

 madreporite. Animals of any kind that can be seized serve iorfood, 

 and usually the stomach can be extruded to envelop and digest prey 





Fig. 417. Echinaster sentus, in the act of devouring a mussel. 

 From Shipley and MacBride. mad. madreporic plate. 



which are too large to be swallowed. Some species clasp bivalves 

 with their arms (Fig. 417) and pull them open with the tube feet 

 so that the everted stomach can be applied to the soft parts of the 

 mollusc. 



In each interradius a stiff septum projects into the perivisceral 

 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 

 Tiedemann's 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 



