iv] MODES OF NUTRITION 91 



hold a mussel, say, and then by means of long- 

 continued traction they cause the latter to open its 

 shell when the starfish protrudes its stomach and 

 sucks away the soft parts of the mollusc. In the 

 larger animals there is however very little trituration 

 of the food in the mouth and hardly at all any 

 insalivation. 



The processes of digestion and assimilation are 

 probably the same in the fishes and invertebrates as 

 in the mammals, but one must not be too dogmatic 

 on this point for very little exhaustive investigation 

 has been made with regard to the digestive processes 

 of the larger invertebrata. The broken-down food 

 contains, in addition to its indigestible skeleton the 

 ' proximate food-stuffs ' of the dietetic text-books ; 

 that is proteid (which is the substance of flesh), fats, 

 and carbohydrates (starches, sugars and gums). The 

 general composition of the food of the marine animals 

 is therefore similar to that of the terrestrial carnivores 

 but it must usually contain a larger proportion of 

 non-assimilable substance, in the shape of the 

 calcareous or horny shells and carapaces of molluscs 

 and Crustacea. The herbivores of the land are 

 represented in the sea by a few fishes such as the 

 mullets which ' graze' on the alga-covered stones 

 of the sea-bottom, and by the plankton feeders. The 

 food of the pasture-feeding or frugivorous land 

 animals contains a relatively large proportion of 



