iv] MODES OF NUTRITION 111 



frequently changed ; and we therefore say that it 

 must receive abundant supplies of oxygen for the 

 purposes of respiration. But the whole object of 

 respiration is the oxidation of the substance of the 

 body in order to yield the animal the energy it 

 requires, and if respiration continues there must be 

 a continual wastage of the tissues or reserve sub- 

 stances of the body. This waste is either replenished 

 by the assimilation of food, or the animal must lose in 

 weight, having used up its own body substance. Now 

 the loss of tissue due to respiration can be calculated 

 from the amount of carbonic acid excreted and it 

 may be greater than can be accounted for by the 

 loss of weight of the animal. In such a case as this 

 the animal must have nourished itself in the manner 

 of a parasite saprozoically, by the absorption of 

 dissolved food matters. 



Why then should an alimentary canal have been 

 evolved among marine animals if they can nourish 

 themselves by absorbing food from solution in the 

 sea- water? In attempting to answer this question 

 we will assume that primitive marine organisms were 

 microscopic in size and that they nourished them- 

 selves by obtaining their food-stuff from solution in 

 the water, just as a peridinian does at present obtain 

 its nitrogenous food-stuff. It gets its carbon food 

 by building it up from the carbonic acid which it 

 also absorbs from solution, but it cannot do this 



