CH. X] THE CONDITIONS OF LIFE IN THE SEA 221 



Modes of nutrition among marine organisms. We have 

 distinguished between two great classes of food-stuffs in the sea ; 

 (1) those which are contained in the living bodies of animals and 

 plants, and which we have called proteids, carbohydrates and fats ; 

 and (2) mineral salts, organic acids, and other relatively simple 

 chemical compounds. Corresponding to this grouping of the 

 food-stuffs we should expect to find a corresponding grouping of 

 organisms into consumers (utilising the organised food- stuffs), 

 and producers (utilising the dissolved inorganic food-stuffs). We 

 do indeed find this division, but the line of demarcation in the 

 sea is not so clear as on the land. 



Holozoic organisms. These are they which can utilise as 

 food only the proteids, carbohydrates, and fats, which have already 

 been built up within the bodies of other organisms. The creatures 

 of the sea which exhibit a holozoic mode of nutrition we call 

 animals. All marine mammals, reptiles and fishes feed in this 

 manner, and among the invertebrata the greater number are also 

 predatory animals. As a general rule all animals in the sea 

 exhibit a preference for some particular food-organism, or group 

 of such. Porpoises devour fishes. Most fishes are carnivorous, 

 thus the cod feeds by preference on other fishes and Crustacea, 

 though at a pinch they are not fastidious. Whiting, turbot and 

 brill are also fish-eaters, but the haddock seems to prefer inverte- 

 brata, such as echinoderms and Crustacea. Some flat-fishes, such 

 as the plaice and flounder, feed predominantly upon molluscs, such 

 as the cockle, mussel, and other lamellibranchs (Tellina, Mactra, 

 Scrohicularia, &c.), but others, like the dab, are almost omnivorous, 

 and eat anything from a fish to a zoophyte. The sole feeds on 

 worms. Some of the molluscs, like the cuttlefishes and whelks, 

 are carnivorous. So also are most of the Crustacea, which are 

 generally garbage eaters, and feed upon the dead bodies of fishes 

 and other animals. This is the case with the crabs, lobsters, and 

 amphipods. Echinoderms, such as the starfish, include many 

 carnivorous species among their classes; thus the starfish may 

 be a most formidable enemy to the mussel and other molluscs. 

 The sea-anemones are also, to some extent, carnivorous animals. 



All these are examples of marine animals which are distinctly 

 predatory in their habits, seeking and devouring other fairly large 

 animals. A further category of carnivorous animals in the sea 



