STUDIES IN ANIMAL LIFE. 



15 



us as a text from which profitable lessons may be 

 drawn. We snip out a portion of its digestive tube, 

 which, from its emptiness, seems to promise little ; 

 but a drop of the liquid we find in it is placed on 

 a glass slide, covered with a small piece of very 

 thin glass, and brought under the microscope. Now 

 look. There are several things which might occu- 

 py your attention, but dis- 

 regard them now to watch 

 that animalcule which you 

 observe swimming about. 

 What is it ? It is one of the 

 largest of the Infusoria, and 

 is named Opalina. When 

 I call this an Infusorium I 

 am using the language of 

 text-books ; but there seems 

 to be a growing belief among 

 zoologists that the Opalina 



Fig. 1 Opalina Ranaeum. 



A, front view ; B, side view — mag- 

 nified. 



is not an Infusorium, but the 



infantile condition of some 

 worm {Distoma ?). However, it will not grow into 

 a mature worm as long as it inhabits the frog; 

 it waits till some pike or bird has devoured the 

 frog, and then, in the stomach of its new captor, it 

 will develop into its mature form — then, and not 

 till then. This surprises you. And well it may ; 

 but thereby hangs a tale, which to unfold — ^for the 

 present, however, it must be postponed, because the 

 Opalina itself needs all our notice. 



