PARASITIC NUTRITION 121 



The presence of numerous nuclei in Opalina is a fact 

 worthy of special notice. The majority of the organisms 

 we have studied are uninucleate as well as unicellular : the 

 higher animals and plants we found (Lesson VI.) to consist 

 of numerous cells each with a nucleus, so that they are 

 multicellular and multinucleate : Opalina, on the other 

 hand, is multinucleate but unicellular. An approach to 

 this condition of things is furnished by Stylonychia, which is 

 unicellular and binucleate (Fig. 23, A), but the only organisms 

 we have yet studied in which numerous nuclei of the ordi- 

 nary character occur in an undivided mass of protoplasm are 

 the Mycetozoa (p. 52), and in them the multinucleate con- 

 dition of the plasmodium is largely due to its being formed 

 by the fusion of separate cells, while in Opalina it is due, as 

 we shall see, to the repeated binary fission of an originally 

 single nucleus. 



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 an internal parasite, or endo- 

 parasite, having the frog as its host. The intestine contains 

 the partially digested food of the frog, and it is by the ab- 

 sorption 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 diges- 

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

 essence of internal parasitism : an organism exchanges a free 



