66 ECONOMIC ZOOLOGY 



then turn the direction of their operations so as to work with the grain, 

 Fig. 45. As the worm increases in size it, of course, enlarges the di- 

 ameter of its hole which thereby becomes conical in shape, as shown in 

 Fig. 45. The much elongated body and siphons, Fig. 45, ss, extend 

 posteriorly to the opening of the tube and the latter serve to conduct a 



FIG. 47. A chiton. Xi. 



stream of water to the animal (just as in ordinary Lamellibranchs) 

 from which food is extracted in the form of microscopic organisms. At 

 the base of the siphons are two small calcareous bodies known as 

 pallets, Fig. 45, p, which serve to close the opening of the burrow against 

 an enemy or to retain the water should the low tide leave the wood 

 exposed. The method of boring of ship-worms has been 

 a much debated question; it is apparently accomplished 

 by the rasping action of the posterior edges of the valves 

 as they are rocked back and forth by the alternate 

 action of the anterior and posterior adductor muscles, 

 the shell being pressed against the end of the burrow 

 FIG. 48. by the cupping action of the foot. The particles of 

 wood that are rasped away are ingested and thus 

 Dentaiium. passed out of the burrow, but whether the animal de- 

 rives any nourishment from them is a debated question. 

 While the introduction of steel ships and the sheathing of wooden 

 ones with metal have largely diminished the havoc wrought by these 

 molluscan pests, still the destruction of small boats and especially of 

 docks and piles, particularly in the tropical and subtropical waters, is 

 often a serious matter. Various preventive methods have been tried, 



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