i8o THE BIOLOGY OF THE SEA-SHORE 



into (i) those which are contained in the tissues of living 

 animals and plants ; and (2) mineral salts, organic acids, 

 and other relatively simple compounds which occur in 

 solution in the sea-water. The first class of food is utilised 

 by animals and the second class by plants. This is a 

 statement which, though essentially accurate, requires a 

 certain amount of qualification. We must make allowance 

 for saprophytic and even saprozoic modes of nutrition. 

 Saprophytes are, of course, plants which derive their food 

 not from the simple substances just mentioned, but by 

 fermenting organic materials, such as sugars, occurring in 

 the soil or in the bodies of other plants. Familiar terrestrial 

 saprophytes are the moulds, yeasts, and fungi. These 

 occur in the sea as well, but not so abundantly as on land 

 (Johnstone, 1908). The term saprozoic is applied to a 

 similar method of nutrition when practised by animals ; 

 as in the case of certain protozoa or — an example of a 

 different nature — of forms such as Convoluta which appa- 

 rently utilise the products manufactured by the activity of 

 plant cells living symbiotically in their tissues (see p. 150). 

 These exceptions, however, do not really weaken the 

 distinction which it is customary to make between plants 

 as the producers and animals as the consumers. Quite 

 different would be the case if the theory put forward by 

 Piitter were proved to be sound, viz, that marine animals 

 obtain much of their food in the form of organic compounds 

 in solution in the water. Further attention will be given 

 to this important question later on. 



The Nutrition of Marine Plants. — Plants in the sea 

 belong to one or other of two great classes : they may be 

 either fixed or floating. The fixed plants are confined to a 

 relatively narrow belt fringing a land mass, whereas the floating 

 plants are universally distributed and, consequently, in the 

 economy of the sea in general, of far greater importance than 

 the seaweeds proper. In the case of the tidal area the im- 

 portance of the two classes of plants is approximately equal. 



All marine plants, however, whether fixed or floating, 

 obtain their nourishment in precisely the same way, viz. by 



