162 Annals Entomological Society of America  [Vol. XIV, 
To these workers it was no longer sufficient to explain the 
occurrence of a tapeworm as due to spontaneous generation 
from the slime or mucus of the intestine or from the digested 
food. As early as 1776 Pallas had called attention to the fact 
that within the little bladders or cysts which occurred within 
the liver tissue of the mouse there was a jointed worm whose 
head structures were wholly comparable with those of tape- 
worms to be found in the intestine of cats. In 1851, the prob- 
lem was solved by Ktichenmeister who proved by experimental 
feedings that similar bladders from the viscera of rabbits 
developed into complete tapeworms when fed to dogs. This 
introduction of the experimental method is justly regarded as 
the basis of our modern conceptions of the development of 
internal parasites. 
Parallel with and in some important conceptions preceding 
this discovery of the typical life cycle of a tapeworm was the 
interpretation of the still more complicated life cycle of the 
liver fluke. So universally is this form used in class work that 
we forget the marvel of the story. The steps that led up to its 
solution are rarely mentioned: 
Baker, Adams, and other early microscopists had figured 
little tadpole-like organisms which they found rarely in pond- 
water, In ly73; O; F: Muller constituted tor these the sents 
Cercaria, which in the light of his times he naturally placed 
among the Protozoa. It seems to have escaped notice that 
before 1680 our old entomological friend, Swammerdam had 
found similar organisms and what we know today as rediz, in 
the snail and had figured them. 
In his work on the anatomy of ‘‘the wonderful viviparous 
chrystalline snail,’’ as he calls Paludina, he tells us that the 
uterus of the snail ‘“‘I immediately met under its upper coat 
which it has in common with the coat of the verge, a congeries 
of oblong little parts (Fig. vii, a) which were very numerous, 
and differed somewhat in their length, figure and thickness, and 
when I removed them from their places, I found they were all 
alive, and were so many living little worms, as there appeared 
particles of that sort. On the inside of these worms was seen 
an oblong transparent ash-gray colored furrow or ridge. When 
I began to dissect one of these worms, two, nay three, and 
sometimes four inclosed worms of the same kind issued forth 
