124 Ol'ALINA i.Kss. 



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 

 Hfe, burdened witli 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 

 ])arasite like Opalina and the saprophytic nutrition of a 

 monad (p. 39). 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, 

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

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

 mode of multiplication which shall serve as a means of dis- 

 persal, or in other words, enable the progeny of the parasite 

 to find their way into the bodies of other hosts, and so start 

 new colonies instead of renraining to impoverish the mother- 

 country. 



Opalina multiplies by a somewhat peculiar process of 

 binary fission : an animalcule divides in an oblique direction 

 (Fig. 24, d), and then each half, instead of growing to the 

 size of the parent cell, divides again transversely (e). The 

 process is repeated again and again (f), the plane of division 

 being alternately oblique and transverse, until finally small 

 bodies are produced (g), about oV^aV "''i'^''- '" length, and 

 containing two to four nuclei. 



If the parent-cell had divided simultaneously into a num- 



