THE ANIMAL PARASITES 617 
ing its completion. This information is perhaps obtained 
most readily in experimentation upon the role of lower 
animal forms in the pathogenesis of disease but where 
reliable evidence is lacking, help may be had by compari- 
son with others in the same taxonomic group. Undoubt- 
edly systematic classification will go far to help solve 
many of these riddles. 
Pathogenicity of Animal Paeasites in Genekal. 
The first question which arises in this connection con- 
cerns the actual ability of animal parasites to produce 
disease in mid animals. At once it will be seen that this 
must be a relative matter, for no one on one side would 
contend that every symbiont in an animal is harmful — 
parasites sensu stricto — nor on the other that none could 
possibly be, i.e., that all are always commensals. It is 
evident that the issue boils down to questions as to the 
extent to which they are harmful. Before attempting 
the answer let us consider the means by which the para- 
sites may conceivably produce disease. 
Modes of Disease Production (Pathogenesis). 
The medical reader is familiar enough with the patho- 
genic powers of some animal parasites, but may be suffi- 
ciently interested to glance over specific wild animal 
instances illustrating them while they are being listed for 
those less familiar with this subject. 
1. Mechanical, Obstruction. 
I refer here particularly to simple blockage of normal 
body passages as the result of bulk or mass. This occurs 
more commonly in the intestines than elsewhere on ac- 
count of the greater frequency, greater numbers and 
larger size, in general, of parasites inhabiting this tract. 
Thus, we have recorded a liothrix {Liothrix luteus) (2) 
where the combination of a small host and consequently 
(2) Phila. Zool. Soc. Rep., 1920, p. 28. 
40 
