Molluscs 143 



by the damage done by this mollusc to 

 the wooden piles supporting the dykes. 

 Despite all kinds of up-to-date pre- 

 cautions the " ship worm " continues to 

 undermine harbour piles, breakwaters, 

 etc. 



A number of strange molluscs have the 

 shell reduced to a mere horny disc 

 covered by the mantle, which is divided 

 into two flaps. Some, such as the " Sea 

 Hare ' J which, when irritated, exudes a 

 purple dye are of large size, certain 

 tropic species measuring nearly two feet 

 in length. The group includes the 

 Pteropodsihe " Sea Butterflies "in 

 which the " foot/' developed into a pair 

 of side fins, flaps through the water with 

 a hurried action suggestive of a moth. 

 They swarm in the Arctic and Antarctic 

 regions, forming one of the chief items 

 in the whale's diet. Many of these 

 naked molluscs are amongst the most 

 beautiful of aquatic animals. When 



