INTRODUCTION 33 
should be changed frequently under the best conditions 
since occasionally one will find groups doing badly until 
moved. Moreover the ground becomes contaminated 
mth parasites such as esophagostomum and heterakis, 
infestation with which while not very serious in itself, 
may lead to fatal infection with bacteria. 
The effect of animal parasites upon the morbidity and 
mortality of wild beasts and birds in captivity is by no 
means clear, and Doctor Weidman and I are inclined to 
be sceptical, with certain reservations of course, of their 
great importance in the death rate. Doctor Weidman has 
kindly agreed to contribute a chapter upon the general 
distribution of protozoal and metazoal parasites with a 
summary of their probable pathogenic importance. 
The groups known to have a decided pathological 
power might be divided into the toxic, the tumor formers 
and the mechanically obstructive ; certain parasites have 
properties placing them in two of these classes. The first 
group comprises the hemosporidia and hemogregarines, 
the uncinaria and some of the cestodes, forms which pro- 
duce hemolysis and hemorrhages with varying grades of 
anemia. The importance of this group is shown chiefly 
among the Aves, in which high grades of anemia are 
occasionally met from malarial infections, but cats and 
dogs or even herbivores also frequently suffer from 
hookworm. The tumor-producers are chiefly echinococ- 
cus worms, the cysts of which may grow large enough to 
occupy nearly the entire abdomen. A certain grade of 
anemia and general ill health accompany this hydatid 
disease, partly the result of a toxin and partly by damage 
to important viscera. Those parasites which obstruct 
mechanically do so by their own bulk or by an accom- 
panying inflammation, incited by them as foreign bodies 
or by bacteria which have gained entrance at the irritated 
point. This is exempUfied by the enormous collection of 
nematodes sometimes found in reptiles (a pailful was 
removed from a python) and by the tightly coiled or 
