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 larvz 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 larve of 
Flies thrive on living Snails does not seem quite 
certain. Melanophora helicivora appears to be para- 
sitic in France on Helicella conspurcata. In North 
America Sarcophaga helicis has been bred from Poly- 
gyra thyroides, and Cynomyia mortuorum from dead 
Snails. Wandolleckia achatine 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 
