MOLLUSCS. 



207 



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 

 animal can burrow below the 

 surface and still obtain its nec- 

 essary 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 (fig. 45). 



The great majority of bi- 

 valve molluscs belong here, but 

 there are comparatively few of 

 general interest. The largest 

 of all clams, the giant clam 

 of the East Indies, with shell 

 sometimes weighing over 300 

 pounds, belongs 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, destroying 

 the piles of wharves, the bot- 

 toms of boats, etc. Their bur- 

 rows run to long distances, but 

 all their food and water must 

 be drawn in through the si- 

 phons. One great inundation in Holland at the begin- 



FIG. 45. Long clam (Mya are- 

 naria) buried in the mud. The 

 arrows show the currents in 

 the siphons. 



