326 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



make use of nourishment in solution. The question is not, 

 however, on what substances marine animals can feed, 

 but on what they do feed under natural conditions. With 

 regard to this first head, which is a biological question, 

 most marine animals are known to take in some solid food, 

 and the presence of an alimentary canal with digestive 

 glands, the structure of appendages or other apparatus 

 for catching food, and the habits of the animals, can only 

 be satisfactorily explained on the assumption that solid 

 food is necessary. Putter, however, points out that it has 

 often been very difficult, or even impossible, to trace any 

 food matter in the alimentary canal or other internal 

 cavities of aquatic animals, and finds it difficult to under- 

 stand how crustacea living iuside sponges can obtain solid 

 food in the filtered water which reaches them. He also 

 states (this will be referred to below) that the amount of 

 solid food necessary for many marine animals is quite 

 beyond their powers of catching. It must be remembered, 

 however, that the absence of food in the alimentary canal 

 is only negative evidence, and we know too little about 

 the bionomics of many of these forms to draw far-reaching 

 conclusions. It is possible, however, that the solid food 

 may be insufficient in quantity to meet the demands of 

 the animal. Putter has calculated the amount of food 

 required daily for several kinds of marine animals by 

 measuring the quantity of oxygen used in 24 hours, and 

 then knowing the composition of the plankton, the 

 number of planktonic forms necessary for food can be 

 calculated. 



One of us* has shown that the alimentary canal of 

 Copepoda contains diatoms, peridinians and flagellates. 

 In fact, the chief food appears to be flagellates and small 



* Dakin, "Notes on the Alimentary Canal and Food of the Copepoda," 

 Internationale Revue der gesamten Hydrobiologie und Hydrographie, Bd. I, 

 No. 6, I. pp. 772-782. 



