1290 THE STORY OF THE UNIVERSE 



dom or never by lamplight; and keep the unoccu- 

 pied eye open. With these precautions you may 

 work daily for hours without serious fatigue to the 

 eye. 



Now where shall we begin? Anywhere will do. 

 This dead frog, for example, that has already been 

 made the subject of experiments, and is now await- 

 ing the removal of its spinal cord, will serve 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 occupy your 

 attention, but disregard 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 In- 

 fusorium I am using the language of text-books; 

 but there seems to be a growing belief among zo- 

 ologists that the Opalina 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 sur- 

 prises 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. 



