XIV] 



ASTERIAS 



331 



which the coelom occupies, and as this is filled with incompressible 

 fluid, the stomach must be pressed out. After some time has 

 elapsed the star-fish relaxes its hold and it is then seen that the 

 shell of the mollusc is completely empty and as clean as if it had 

 been scraped with a knife. It was long a puzzle how the star-fish 

 succeeded in forcing its victim to relax its muscles and allow the 

 valves to open. It was supposed that the stomach secreted a 

 paralysing poison, but it has been conclusively proved that this is 

 not the case, but that the star-fish drags the valves of its victim 

 apart by main force, often actually breaking the adductor muscles. 

 By assuming the bunched up position shown in Figure 152 the 



FIG. 152. Echinaster sentus, in the act of devouring a mussel. 

 1. Madreporic plate. 



star-fish is enabled to use the tube-feet of opposite arms to pull on 

 the valves of the victim in opposite directions. The pull exercised 

 by the suckers is not nearly strong enough to open the valves at 

 once, but the star-fish has staying power and eventually the mussel 

 is slowly forced open. The cells lining the stomach include a large 

 number of goblet-cells (v. p. 151) swollen by drops of clear fluid; 

 those of the pyloric sac, on the other hand, present a different 

 appearance. These are full of minute granules and recall the 

 appearance of the cells in other animals which contain the active 

 digestive principle. Hence it seems reasonable to suppose that 

 the mussel is digested by the secretion of the pyloric sac and its 



