STUDIES IN ANIMAL LIFE. 29 



plained rather than fly to spontaneous generation. 

 And they were right. The mystery has at length 

 been cleared up.* I will endeavor to bring togeth- 

 er the scattered details, and narrate the curious 

 story. 



Under the eyelids of geese and ducks may be 

 constantly found a parasitic worm (of the Trematode 

 order), which naturalists have christened Mbnosto- 

 mum mutabile Single-mouth, Changeable. This 

 worm brings forth living young, in the likeness of 

 active Infusoria, which, being covered with cilia, 

 swim about in the water as we saw the Opalina 

 swim. Here is a portrait of one. 



I 



Fig. 4. A, EMBKYO OF MONOSTOMUM MCTABILK. 

 B, Cercaria sac, just set free. 



a, mouth; &, pigment spots; c, sac magnified. 



Each of these animalcules develops a sac in its 

 interior. The sac you may notice in the engraving. 



* By VON SIEBOLD. See his interesting work, Ueber die Band- 

 md-Blasenwurmer . It has been translated by HUXLEY, and ap- 

 pended to the translation of KUECHENMEISTEB on Parasites, pub- 

 lished by the Sydenham Society. 



