OBSERVATIONS ON ENTOZOA. 
899 
forms of disease produced by parasites as coming under the 
general designation of “ evilsfor, in particular instances, 
the presence of Entozoa may constitute a veritable plague, 
in the ordinary and non-superstitious acceptation of the 
term ; and it may be correctly spoken of as such, in the same 
sense as you would so speak of the “ rinderpest,” or of pleuro¬ 
pneumonia. But parasites hold a ground of their own; and, 
as a rule, it may be said that the endemic evils which they 
effect are, for the most part, due to overcrowding. This I 
shall explain more fully by-and-by; however, in the mean 
time, there are other facts of special significance that you will 
do well to bear in mind. 
In the hope that I have made myself intelligible thus far, 
and have conveyed to your minds accurate conceptions of the 
intermutual relations of host and guest, I am encouraged to 
pass on by observing that the Entozoa are, for convenience 
sake, divided into four natural groups or categories, respec¬ 
tively termed Trematoda, Cestoda , Nematoda , and Acantho - 
cephala. The first of these natural divisions, or orders, as 
they are technically termed, embraces the so-called flukes or 
flounders, including the Distomes and their allies. They are 
very closely connected with the Planarians, small creatures 
which are to be found in our ponds and ditches. Most of you 
are probably acquainted with the “ rot” of sheep. That 
disease is due to the presence of the common fluke; and, as I 
shall show you, our knowledge of the pathology of this 
affection is very largely indebted to the labours of Professor 
Simonds. 
In regard to the remaining three natural divisions or orders 
of the Entozoa above referred to, I will in this place only 
further define them to the extent of saying that the group 
Cestoda comprises the tapeworms; the division Nematoda 
embraces the round worms and thread worms ; whilst the less 
important and small group, termed Acantho cephala, includes 
only the thorn-headed parasites, of which we have an inte¬ 
resting representative infesting the pig. More or less in¬ 
structive types of all these orders of Entozoa will come under 
our notice as we proceed to discuss, in succession, the struc¬ 
ture, character, and habits of, as well as the symptoms and 
diseases produced by, the different individual species in¬ 
festing our domesticated animals. 
