120 



LIFE, IN ITS LOWER FORMS. 



perfect animal, supplied with complicated male and female 

 organs, and capable of producing a multitude of fertile 

 eggs, quite independently of all the other segments. The 

 segments that contain mature eggs are usually detached 

 from the rest, and separately expelled from the body of 

 the patient. 



We have hitherto spoken of these different forms of 

 parasitic worms as so many distinct species, for so they 

 have, up to a late period, been considered by naturalists, 

 taking their places in our zoological systems unchallenged 

 under not only specific but distinct generic appellations. But 

 the observations of an eminent German physiologist, Von 

 Siebold, go far to prove that the Cystoid Worms are but 

 the earlier undeveloped stages of the Tcenioid forms, and 

 not only so, but that these larval creatures assume quite 

 different forms and possess different habits according to 

 the kind of animal within whose body they live. That, 

 for example, the microscopic egg or embryo of a Tcenia, 

 evolved in the intestinal canal of a dog or cat, if taken 

 with food into the stomach of a rat, finds its way invariably 

 to the liver, and becomes a Cysticercus, while if it be 

 swallowed by a sheep, it travels by some recondite road to 

 the brain, and is transformed into that parasite so fatally 

 known as producing the " staggers," Coenurus. Let either 

 of these now, in turn, be swallowed by the carnivorous 

 quadruped, and a Tcenia is the invariable result. 



But these facts are so curious that our readers may be 

 pleased to read the observations themselves, as recorded 

 by their learned author : — 



" I was the first to advance, in the second volume of 

 my ' Manual of Physiology,' published in 1844, the state- 



