282 OPALINA CHAP. 



There is no contractile vacuole, and no trace of either 

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

 impossible. 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 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 nutri- 

 ment. 



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 ex- 

 changes 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 in- 



