636 THE INVERTEBRATA 



ambulacral groove runs a double row of large, transversely placed, 

 ambidacral ossicles^ movable upon one another by muscles. Each has 

 a smaller adambulacral ossicle at its outer end. Adambulacral spines 

 stand on the adambulacral ossicles. In Asterias they are long, and 

 bear groups of large pedicellariae of the kind with uncrossed jaws. 

 They can be turned inwards to protect the ambulacral grooves. 



The mouth leads through a short oesophagus (Fig. 441) into a large 

 sac-like stomach, 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 /))'/onc 

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





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Fig. 442. Echinaster sentus, in the act of devouring a mussel. 

 From Shipley and MacBride. mad. madreporic plate. 



of an arm: the epithelium of these secretes the digestive ferments 

 and stores nutriment. 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 

 {or food, and usually the stomach can be extruded to envelop and 

 digest prey which are too large to be swallowed. Some species clasp 

 bivalves with their arms (Fig. 442) 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 



