78 THE LIFE OF THE MOLLUSCA 



cutting the muscle that attaches the operculum to 

 the animal to prevent its closing, after which it 

 proceeds to devour the animal and undergo its meta- 

 morphosis within the shell. Some species of Silpha 

 seize small Helices in their mandibles, and break the 

 shell by throwing their head back and striking it 

 against their prothorax. The larvse of Cochleoctonus 

 vorax prey on different species of Snails. The big 

 Water Beetle (Dytiscus) and other water insects live 

 largely on Water Snails. To what extent larvae of 

 Flies thrive on living Snails does not seem quite 

 certain. Melanophora helicivora appears to be para- 

 sitic in France on Helicella conspuvcata. In North 

 America Sarcophaga helicis has been bred from Poly- 

 gyra thyvoides, and Cynomyia mortuorum from dead 

 Snails. Wandolleckia achatince has been found run- 

 ning over living Achatina in Liberia, but whether 

 they are parasites or not does not appear. 



Small molluscs are swallowed whole and the animal 

 digested out by Sea Anemones and Starfish. The 

 latter, too, is very destructive in Oyster-beds, open- 

 ing the valves either by main force or other means, 

 and then, everting its stomach and passing it between 

 the valves, it digests and absorbs the mollusc. 



Among molluscan parasites the most abundant 

 belong to stages in the development of Platyhelmin- 

 thian Worms, which complete the cycle of their 

 existence in some other animal, such as Birds, Fish, 

 Frogs, etc. The history of these is completely 



