CHAPTER 6 



Feeding and Digestion 



FEEDING MECHANISMS 



T 



■ N THE ANIMAL KINGDOM as 3 wholc there have evolved mechanisms 

 I enabUng various animals to deal with almost every conceivable 

 m __ source of food materials. For the most part there is no correlation 

 between the types of food materials utilized and the major groups in the 

 natural scheme of classification. Practically all of the major divisions in the 

 animal kingdom have species feeding on a wide variety of foods and using 

 correspondingly diversified feeding mechanisms. The nature of feeding 

 mechanisms appears to have been determined largely by the habitat and the 

 food materials available to a particular organism. The form and function of 

 the digestive tract, as well, are in most cases definitely correlated with the 

 types of food used and the feeding mechanisms. 



Soluble Food. All natural waters have organic material dissolved in them 

 in concentrations of the general order of magnitude of 10 mg. per liter, the 

 amounts being greater in regions of much organic detritus. There are proteins, 

 some carbohydrates which are principally pentosans, some amino acids and 

 fatty acids, all of which have arisen from dead organisms or as waste products 

 of living ones. Piitter ^'^ was the first to consider the significance of these 

 dissolved substances as food for animals. He believed that he demonstrated 

 them to be important as food, but this conclusion was seriously questioned by 

 many later investigators. The subject has been ably reviewed by Krogh.-^'^ 

 Only a few free-living animals, particularly the protozoans and possibly 

 sponges, appear capable of relying in nature on simple dissolved substances 

 such as amino acids, sugar, and fatty acids as an important portion of their 

 food supply. In the protozoans the substances appear to be absorbed directly 

 through the cell membrane; in the sponges the feeding cells or choanocytes 

 are probably responsible for uptake of the dissolved foods. Direct absorption 

 of simple foodstuffs must be the principal "feeding mechanism" in helminth 

 parasites, such as restodes and acanthocephalans, which possess no digestive 

 tract, and in parasitic protozoans. Where simple compounds in solution are 

 an important food source there must usually be mechanisms for active trans- 

 port across cell membranes since the materials are carried from a low to a 

 higher concentration. The utilization of complex dissolved foodstuffs such 

 as proteins and polysaccharides probably requires digestion either in food 

 vacuoles or in a functional digestive tract. 



There is some evidence that higher animals, such as lamellibranch molluscs, 

 and even fishes and frog tadpoles, are able to absorb and utilize dissolved 

 nutrients from their aqueous environment when these substances are present 

 in rather high concentrations. However, even when high concentrations are 

 present the amounts absorbed are insufficient for normal metabolic require- 



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