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 onlyspecificbut distinct generic appellations. But 
the observations of an eminent German physiologist, Yon 
Siebold, go far to prove that the Cystoid Worms are but 
the earlier undeveloped stages of the Tmnioid 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 Taenia, 
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 Cystkercus, while if it be 
swallowed by a sheep, it travels by some recondite road to 
the brain, and is transformed into that parasito so fatally 
known as producing the “ staggers,” Coenurus. Let either 
of these now, in turn, be swallowed by the carnivorous 
quadruped, and a Taenia 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 : — 
X was the first to advance, in the second volume of 
my ‘ Manual of Physiology,’ published in 1844, the state- 
