350 
ROT IN SHEEP. 
example, when the operculum comes off from the egg; that 
they then become parasitic to some of the creatures which 
are met with in water, and that when so parasitic they have 
the capability of propagating themselves ; that they subse¬ 
quently pass through a series of changes, and again become, 
as I shall show you by analogy, parasitic a second time, when 
they change into the fluke-like form. So that, tracing the 
process all through, we may say that the egg sends forth 
a circular germ which is ciliated, and which has a rotatory 
motion in water: that in this condition it becomes parasitic 
probably to molluscs, small snails, and creatures of this 
description : that when it gets into the body of such animals, 
working its way in just below the skin by the cilia which it 
possesses, it undergoes a perfect change, becoming some¬ 
thing like a chrysalis, in which condition it propagates itself: 
that the creatures immediately coming from it reach a certain 
order of development in the snail: that they then escape, 
swim about in water, and, after a certain time, again become 
encased, and again become parasitic to the snail, in order 
that they may reach their highest form of development. All 
this is explained in one of the diagrams here exhibited. 
Putting aside the eggs as representatives of distomata, w r e 
go back to a peculiar organism, as an analogy, which is often 
met with in small water snails, slugs, and the like, imbedded 
underneath the common integument in different parts of the 
body. It has been called a cercaria-sac, so designated from 
the creature which issues from it possessing a tail, with which 
it swims about in water. In this sac we have vital germs— 
very small granular matter—lying beside a kind of intestinal 
canal, which arc sent off as gemmae or buds. We find this 
budding process goes on, and that a considerable number of 
these germs are ultimately set at liberty. Within the sac 
these develop into the true cercaria, and when they have 
arrived at a certain period of development they burst through 
the sac, and swim freely about in w^ater. [The diagram 
represents the different degrees of development.] You first 
see the small germ, which comes off as a gemma or bud, 
becoming elongated in one part of its body. Subsequently 
this elongates into a tail-like appendage; other organs are 
then added, and in this condition the creature bursts through 
the sac, leaving others to come out of it which are produced 
in the same manner. It swims about in water w r ith a long, 
floating tail, and as such it has been viewed as one of the 
infusoria. Subsequently it curls itself up, and, as a general 
rule, gets in the first instance upon slugs, entering the slimy 
matter which covers them, and imbedding itself in ito Then 
