282 OPALINA . CHAP. 



There is no contractile vacuole, and no trace of either 

 mouth or gullet, so that the ingestion of solid food is impos- 

 sible. The creature lives, as already stated, in the intestine 

 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 pro- 

 cess 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 parastic 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. 257). 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 Opalinae simply went on multiplying, 

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

 would soon outgrow the means of subsistence : moreover, 



