FEEDING OF MARINE ANIMALS 205 



much more powerful than the tube-feet, yet they soon tire, 

 and finally the steady pull of the starfish conquers and the 

 shell begins to gape. As soon as this happens the starfish 

 extrudes its stomach through its mouth and forces it into the 

 opening (Fig. 46), where it first probably poisons and later 

 certainly digests the soft body of the prey. It is this habit 

 which makes the starfish the greatest pest on oyster and 

 mussel beds. Starfish can even attack the apparently 

 vulnerable sea urchin by forcing their stomachs down its 

 throat apparently quite regardless of the teeth. 



Many worms are carnivorous, seizing their food by means 

 of horny jaws and usually swallowing it whole. Whelks and 

 their relatives live on carrion, often dead or dying bivalves ; 

 their mouths are situated at the end of long probosces 



Fig. 46. — Diagram showing how a Starfish 

 feeds on its prey. (Modified after Hirsch.) 



which can be extruded for a considerable distance, and in 

 this manner the inside of a mussel can be cleaned out, the 

 flesh being scraped into the mouth by the radula. Many 

 similar animals are able to attack living bivalves by boring 

 through their shells either mechanically by means of the 

 radula, or else chemically, the animal pouring out strong 

 acid which eats through the calcareous shell. As soon as 

 the opening is made the long proboscis with which all these 

 carnivorous snails are equipped is pushed into the soft 

 flesh, which is probably poisoned and then very quickly 

 consumed. Others again open the shells of oysters by 

 moving on to them and waiting until the oyster opens when, 



