272 OPALINA CHAP. 



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 remaining to impoverish the mother 

 country. 



Opalina multiplies by a somewhat peculiar process of 

 binary fission : an animalcule divides in an oblique direction 

 (Fig. 7 1 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 yd^sV "^'^i- i" length, and 

 containing from two to four nuclei. 



If the parent cell had divided simultaneously into a num- 

 ber of these little bodies the process would have been one of 

 multiple fission (p. 254) : as it is, it forms an interesting link 

 between simple and multiple fission. 



Opalina ranarum multiplies in this way in the spring — i.e. 

 during the frog's breeding season. Each of the small pro- 

 ducts of division (g) becomes encysted (h), and in this 

 passive condition is passed out with the frog's excrement, 

 probably falling on to a water-weed or other aquatic object. 

 Nothing further takes place unless the cyst is swallowed by 

 a tadpole, as must frequently happen when these creatures, 

 produced in immense immbers from the frogs' eggs, browse 

 upon the water-weeds which form their chief food. 



Taken into the tadpole's intestine, the cyst is burst or 

 dissolved, and its contents emerge as a lanceolate mass of 

 protoplasm (i), containing a single nucleus and covered with 

 cilia. This, as it absorbs the digested food in the intestine 

 of its host, grows, and at the same time its nucleus divides 

 repeatedly (k) in the way already described, until by the time 

 the animalcule has attained the maximum size it has also 



