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. I.-OPALINA RANARCM. i s not an Infusorium, but the 

 A, front view; B, side view mag- . . 



nified. 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. 



