XI PARASITISM 123 



the spindle (4, 5), and the nucleus becomes constricted (5) 

 and finally divides into two (6). 



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 : Opalina, on the 

 other hand, is multinucleate but its protoplasm is undivided, 

 so that it presents a condition of things intermediate be- 

 tween the unicellular and the multicellular types of structure : 

 it is most suitably described as non-cellular. An approach to 

 this condition is seen in Stylonychia, which is unicellular 

 and binucleate ; but apart from Annvba qitarta, the only 

 organisms we have yet studied in which numerous nuclei of 

 the ordinary character occur in an undivided mass of 

 protoplasm are the Mycetozoa (p. 52), and in them the 

 multinucleate condition 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 



