239 



cally uiK'hanged, 1)(.'ing passed througli the digestive tract simply t:)ecaiisc there is 

 no other convenient method of disposing of them. Certain near relati\es of Teredo 

 {Pholas, Zirfaea and Saxicava) bore in such substances as sandstone, Hmestone, shale, 

 marl and gneiss — materials which cannot be supposed to yield them any nourishment; 

 3,'et these indigestible borings may be found in considerable (juantities in the digesti\e 

 tract of the rock borers. The principal constituents of wood, cellulose and lignin, are 

 notably resistant to digestive action, so that they pass through the digesti\e tract of 

 many animals entirely unchanged by the enzymes there encountered. Further, wood 

 that has passed through the digestive tract of Teredo is shown by microscopic study 

 to have retained its cellular structure, as seen in fig. 91, and some investigators claim 

 to have identified the kind of wood by its odor and color. On the basis of these con- 

 siderations, the statement has been commonlv made that Teredo derives no nutriment 



"1 T f 



Fig. 90. Split section of pile shown in fig. 89. 



from the wood, but subsists, as do other lamellibranchs, on a diet of plankton organ- 

 isms filtered from the water which is continually passing in and out through the 

 siphons. 



Recently this problem has been approached from the biochemical point of view, 

 and rather definite conclusions have been arrived at. Harington (1921), working at 

 the Plymouth Laboratory, has shown that a certain enzyme or enzymes extracted 

 from the excised livers of Teredo norvegica will act on sawdust to produce glucose, a 

 substance which is readily absorbed into the blood and consumed in the tissues of 

 animal organisms. His results are not as conclusi\-e as might be desired, since the 

 liver extract failed to reduce pure cellulose in his experiments. It is possible that the 

 action on the sawdust was limited to the small amounts of hemicelluloses present, 

 which are much more easily broken down than is pure cellulose. 



