ACEPHALA. 



263 



ORDER II. SIPHONATA. 



In these the margins of the mantle have grown together 

 posteriorly into a double tube or siphon, and accordingly as 

 this siphon is developed the an- 

 imal can burrow below the sur- 

 face and still obtain its necessary 

 supplies of water and food; for 

 these tubes can reach the surface, 

 and through them there is a 

 continual flow of water inward 

 through the ventral, outwards 

 through the dorsal, passage. 



The great majority of bivalve 

 molluscs belong here, but there 

 are comparatively few of general 

 interest. The largest of all 

 clams, the giant clam of the East 

 Indies, with shells sometimes 

 weighing over 300 pounds, be- 

 longs here, as do the quahog and 

 the long clam, which are used as 

 food. One of these forms, the 

 Teredo or ship-worm, is a serious 

 pest, as it bores in wood, destroy- 

 ing the piles of wharves, the 

 bottoms of boats, etc. Their 

 burrows run to long distances, 

 but all their food and water 

 must be drawn in through the l %U8iiCI^BSF-fB 

 siphons. One great inundation e s Y s h ons W the currents m 

 in Holland at the beginning of 



the last century was directly due to the borings of these 

 forms, 



