iv REPRODUCTION 283 



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 (compare p. 274), 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 improverish the mother-country. 



Opalina multiplies by a somewhat peculiar process of 

 binary fission : an animalcule divides in an oblique direction 

 (Fig. 73 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 -^ -$ mm. in 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 numbers 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. Conjugation occurs, and as the young Opalina absorbs 

 the digested food in the intestine of its host, it grows, and 



