108 LIFE IN THE SEA [CH. 



examine the contents of the alimentary canal of 

 these animals we nearly always find that it contains 

 hardly anything but fine sand and mud particles. 

 There are never many food organisms, such as diatoms. 

 It has been said that these animals may feed on such 

 organisms as infusoria which possess no shells and 

 which could, therefore, hardly be identified in the 

 contents of the intestine, but we do not find many 

 such if we examine the water in which the molluscs 

 live. Sometimes indeed we do find that the intestine 

 contains large quantities of green matter which 

 probably consists of algal spores, but this is ex- 

 ceptional. If, however, we remember that it is 

 essential for the good condition of such molluscs as 

 these that they should live in a part of the sea 

 where there is plenty of fresh water flowing down 

 from rivers, then we receive the suggestion that 

 probably the food may be in solution, for the water 

 of rivers flowing into the sea contains more of 

 dissolved organic compounds than does sea-water. 



Then it can be determined in many cases that 

 the quantity of planktonic organisms contained in 

 the plankton is too small to account for the observed 

 metabolism of an animal. A mussel, for instance, 

 may very easily grow from two inches in length to 

 two and a half inches in the course of a single 

 summer, and it was easy to show in one such series 

 of cases that this growth represented an average 



