iv] MODES OF NUTRITION 107 



from an alimentary canal should always be regarded 

 as a possibility of nutrition. 



If we assume that marine animals living in the 

 open nourish themselves exclusively by capturing 

 and digesting solid food we immediately get into 

 difficulties. For it is often difficult to convince 

 oneself, by examination of the contents of the ali- 

 mentary canal, that sufficient food organisms for the 

 apparent needs of an animal have been ingested. 

 We hardly find anything in the intestine of a lug- 

 worm except sand which does not differ from that 

 in which the animal lives. It burrows by eating 

 the sand in the same way as an earthworm burrows 

 by eating the soil, and we suppose that its food is 

 contained in the sand that passes through its body. 

 But if its food consists of microscopic organisms there 

 are very few such in the sand beneath the superficial 

 layers. Many organisms have indeed been identified 

 from the contents of the intestine of the lugworm 

 but it is always possible to regard these merely as 

 residues which are contained in the sand, and it does 

 not appear to be probable that the worm can obtain 

 enough food-stuff in this shape from the sand which 

 it ingests. Just the same difficulty meets us in the 

 cases of cockles and mussels which live on the sea- 

 bottom and which have been supposed to obtain 

 their food from the plankton contained in the water 

 which passes through their shell cavities. If we 



