CHAPTER 6 



DIGESTION 



For many divisions there are in the stomack of severall animals : what 

 number they maintain in the Scarus and ruminating Fish, common 

 description, or our own experiment hath made no discovery. But in the 

 Ventricle of Porpuses there are three divisions. In many Birds a crop, 

 gizzard, and little receptacle before it; but in Cornigerous animals, 

 which chew the cudd, there are no less than four of distinct position 

 and office. 



Sir Thomas Browne, Garden of Cyprus 



INTRODUCTION 



In the preceding chapter we have reviewed various feeding methods 

 encountered among marine animals and we now turn to a consideration 

 of how foodstuffs are digested and absorbed. The earliest holozoic 

 denizens of the seas were probably unicellular forms that ingested their 

 food by phagocytic action. This level of organization is represented by 

 marine rhizopods and flagellates among the protozoa. In this group there 

 is diverse specialization within the boundary of a single cell-equivalent, 

 and intracellular digestive mechanisms are present. Digestion in sponges is 

 essentially the same as in protozoans, since they lack a true gut and capture 

 food particles by means of flagellated choanocytes, which resemble choano- 

 flagellate protozoans. In metazoans a true gut is present, except in certain 

 degenerate and parasitic forms, and it is here that the food is processed and 

 broken down preparatory to assimilation by the animal. In the following 

 account attention will be focused on digestive processes in invertebrate 

 metazoans and lower chordates. Comparative reviews of digestion in these 

 animals have been prepared by Yonge (72), Vonk (66), Barrington (9), 

 Prosser (57) and Mansour-Bek (43). 



FUNCTIONAL DIVISIONS OF THE GUT 



The foodstuffs utilized by animals and the digestive mechanisms which 

 deal with the ingested food show extraordinary diversity throughout the 

 animal kingdom. Every conceivable kind of organic food is exploited and 

 consumed in the sea. In general it may be said that the form of the alimen- 

 tary canal and the nature of the digestive process are correlated with the 

 mode of feeding and character of the food. From a functional viewpoint 

 Yonge (72) has proposed a classification recognizing the following five 

 regions in the gut: (a) reception; (b) conduction and storage; (c) digestion 

 and internal trituration; (d) absorption; (e) formation and transport of 

 faeces. 



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