2 SCIENCE PROGRESS 



They can only utilise as food-stuffs the proteid, carbohydrate, 

 and fat which have already been built up from inorganic material 

 by the producers. Therefore the mass of animals in the sea 

 depends on that of the plants, while the mass of the latter 

 depends again on the amount of the indispensable food-stuffs 

 which stand at their disposal. 



Such a distinction between plants and animals is founded 

 on the mode of nutrition characteristic of each, and is not really 

 invalidated by the existence of organisms which, while they are 

 indubitably animals, yet possess chlorophyll corpuscles in their 

 tissues, and so can elaborate starch from water and carbon 

 dioxide by a process of photo-synthesis. For these chlorophyll 

 corpuscles are entirely alien to the body of the animal which 

 contains them. They originate in the "infection" of the latter 

 by a single-celled alga, and subsequently multiply in the tissues 

 of the animal host, or partner, of the association. We speak of 

 such associations as symbioses, or perhaps cases of parasitism. 

 They are to be observed in many groups of the animal kingdom 

 — protozoa, ccelenterata, flat-worms, echinoderms, polyzoa, and 

 some mollusca. Such compound organisms exhibit two modes 

 of nutrition which proceed simultaneously in the course of 

 their metabolism — that of the holozooic animal, and that of 

 the holophytic plant. 



Although we may make a general distinction between the 

 plant-like and animal-like organisms of the sea, there are 

 nevertheless many species which cannot properly be placed 

 in either category. Many plants are saprophytic, and not 

 holophytic : they obtain their carbon and nitrogen from such 

 complex substances as fatty acids or carbohydrates, and their 

 nitrogen from compounds like urea, guanin, hippuric acid, uric 

 acid, etc., instead of from the very simple substances, carbon 

 dioxide, and nitric acid and ammonia salts. Saprophytes in the 

 sea are usually moulds, yeasts, and fungi, but they are not 

 abundant. The marine bacteria are usually saprophytes ; but 

 they also include many species which exhibit quite special 

 modes of nutrition. Thus the sulphur bacteria can utilise, as 

 a source of energy, sulphuretted hydrogen ; nitrifying bacteria 

 can obtain their carbon and nitrogen from carbon dioxide, and 

 nitrous or nitric acid, and can make use of these substances 

 as food-stuffs without possessing the power of photo-synthesis ; 

 and the denitrifying species can obtain their energy from the 



