RECENT WORK ON MARINE METABOLISM 3 



reduction of the nitrogen acids. Among the true animals there 

 are many which have a mode of nutrition which may be termed 

 saprozooic. These are the infusoria, which live in organic 

 liquids, and the internal parasites which inhabit the cavities 

 of the animal body. Tapeworms possess no alimentary canal 

 and absorb the carbohydrate or proteid, or the digestive pro- 

 ducts of the latter, to be found in the intestine or other part of 

 the body of the host in which they live, by means of their entire 

 body surface, A distomum does possess an alimentary canal ; 

 but there is little doubt that it also absorbs the nutritive matter 

 of the juices of the host by means of its skin. The internal 

 parasites of an animal thus exhibit a mode of nutrition which 

 is very similar to that of a saprophytic plant. 



It is generally assumed in biological textbooks and lectures 

 that, apart from the mode of alimentation exhibited by the 

 internal parasites, all animals feed in much the same way as 

 is witnessed in the higher terrestrial forms. Food organisms 

 are captured and ingested, and digestive processes proceed in 

 the alimentary canal in much the same manner as in the case of 

 the mammal, that is, proteid food substances are hydrolysed, 

 absorbed, and reconverted into the proteid substances specific 

 to the species considered ; carbohydrates are hydrolysed to form 

 diffusible sugars ; and fats are saponified. Further, the current 

 metabolism is usually regarded as similar to that of the warm- 

 blooded animals ; that is, energy is obtained from the oxidation 

 of part of the tissue substance ; carbon dioxide and water being 

 excreted, along with certain nitrogenous residues ; that the 

 amount of oxygen absorbed in respiration is nearly equal to 

 the oxygen which is present in the excretory products ; and 

 that a fairly constant respiratory quotient is the result of the 

 metabolism of the animal. 



Now the consequence of such assumptions is that the com- 

 parative anatomists have described in the invertebrata series 

 of organs which are stated to carry out functions similar to those 

 of the organs with similar names of the warm-blooded animal. 

 The alimentary canal is always an organ in which the food- 

 substances are digested and absorbed. The " hepato-pancreas " 

 of a mollusc or other invertebrate is always a gland in which 

 glycogen is elaborated, and in which enzymes are prepared ; 

 enigmatical structures in connection with the alimentary canal, 

 such as the crystalline style of the mollusca, are reserves of food 



