622 DISEASE IN WILD MAMMALS AND BIRDS 
7. Preclusion of Nutrition. — This must be a very 
unimportant phase of the activity of intestinal parasites, 
when one compares the bulk of food which passes through 
the bowel and the average number of worms present ; and 
the same holds good for some interstitial parasites like 
the adult filariae. Even in amazingly liea\'^" infestments 
of the intestines one will be constrained to dismiss this 
idea when he compares the bulk of parasites with that of 
the host, and recalls what the physiologist terms the 
** factor of safety" inherent in this tract as elsewhere. 
But in the case of blood parasites the matter may be 
different. Here we are concerned with the withdrawal of 
refined foodstuffs — those which have been worked over 
and over by subtle internal metabolic processes ; and we 
are not so sure, especially on recalling the enormous 
numbers of parasites usual to blood infestments, that 
there is the capacity on the part of these internal 
processes to meet increased demands that we count upon 
for the intestinal functions. It is much more serious to 
be deprived of the finished product than of the crude 
because it means the undoing of "digestive" work all 
along the line, from gut to tissue cell. Furthermore, a 
blood infestment guarantees that the parasite has been 
feeding upon and depriving the animal of the precise 
foodstuffs the cells require, and not by any chance upon, 
even in part, intestinal substances that were wastes or 
residues. If we except the blood parasites, then, it seems 
safe to conclude on the whole that the amount of pabulum 
used by parasites is unimportant to the animal. 
Having reviewed the manner in which parasites may 
conceivably be harmful, it is time to return to the question 
of the actual exercise of these powers. 
The older appraisal of parasites in animals, namely 
that they were rather innocent of disease production, was 
suggested by and borrowed from the veterinarian, prob- 
ably being engendered in him by their frequency in what 
appeared to be normal domestic specimens. Yet it is only 
