Ill NUTRITION AND REPRODUCTION 271 



of the frog : it is therefore, like the worms you have probably 

 noticed in the frog's urinary bladder and lungs (pp. 33 and 

 153), an internal parasite, or endoparasite, having the frog 

 as its host. The intestine contains the partially-digested 

 food of the frog, and it is by the absorption of this that the 

 Opalina is nourished. Having no mouth, it feeds solely by 

 imbibition : whether it performs any kind of digestive 

 process itself is not certainly known, but the analogy of 

 other mouthless parasites leads us to expect that it simply 

 absorbs food ready digested by its host, upon which it is 

 dependent for a constant supply of soluble and diffusible 

 nutriment. 



Thus Opalina, in virtue of its parasitic mode of life, is 

 saved the performance of certain work — the work of digestion, 

 that work being done for it by its host. This is the essence 

 of internal parasitism : an organism exchanges a free life, 

 burdened with the necessity of finding food for itself, for 

 existence in the interior of another organism, on which, in 

 one way or another, it levies blackmail. 



Note the close analogy between the nutrition of an internal 

 parasite like Opalina and the saprophytic nutrition of a 

 monad (p. 256). In both, the organism absorbs proteids 

 rendered soluble and diffusible, in the one case by the 

 digestive juices of the host, in the other by the action of 

 putrefactive bacteria. 



The reproduction of Opalina presents certain points of 

 interest largely connected with its peculiar mode of life. It 

 is obvious that if the Opalinse simply went on multiplying, 

 by fission or otherwise, in the frog's intestine, the population 

 would soon outgrow the means of subsistence : moreover, 

 when the frog died there would be an end of the parasites. 

 What is wanted in this, as in other internal parasites, is some 



