22 



sei've for food. Those consist mainly of diatoms and 

 simple floating Algae, and other minute organisms. Small 

 Crustacea and other animal forms seom almost nevor oaten. 

 The particles of v/ood that are rasped away in form- 

 ing the burrow are ingested, as the only r.vjjans o:' trotting 

 rid of them. It has often been debated v^hether they xin- 

 dergo any digestion in the alimentary canal, and I am in- 

 clined to think they contribute something to the nutrition 

 of "Siiip-wonns". Boring I tliink to be a periodical func- 

 tion, perhaps alternating with more active ingestion of 

 food. This is indicated by two facts. In the first place 

 it seems that while the teeth are boing cemented to the 

 anterior edges of the valves , the shell coi-ld not be used. 

 Also, the caecum of the stomach contains mainly particles 

 of v/ood, v/hich indicates that v/hile burrowing, the orifice 

 into it is open. The caecum contains a very large fold of 

 the internal membrane tfif^s. 31-33) and seems so eminently 

 an absorbent structure, that for this reason it seems to 

 me there must be some action on the particles of wood '.vith- 

 in it. 



