BEARING OF PHYSIOLOGY OF PARASITIC NEMATODES ON THEIR CONTROL.
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tested knowledge of their physiology. We know even less about their physio-
logical relationships with their hosts, upon which our measures of control and
treatment must depend. Without this physiological knowledge we cannot
understand the pathology of the diseases caused by these or any other parasites ;
without it we cannot even understand the equilibrium which exists in nature
between any host and its parasites. Almost every animal slaughtered for human
consumption, or used for agricultural purposes, harbours many parasites, including
many nematodes, and yet remains healthy and marketable at an economic profit.
Man himself lives harmoniously with many parasites, for we must include among
parasites all the bacteria. If we could understand the physiological basis for
this equilibrium which occurs so often in nature, we could at least try to maintain
farm animals in it, and this might even be sufficient for economic purposes.
A complete understanding of the physiological relationship between the
host, on the one hand, and the complex formed by all its parasites, on the other,
is of course an ideal at present far beyond our reach. We can only work towards
it, realising the great complexity of the problems involved, and hoping that, as
our knowledge extends, we may be able to use the struggle for existence among
the various parasites present to the advantage of the host which harbours them,
in much the same way as the medical man has used the malarial parasite for the
treatment of general paralysis, or the economic entomologist has encouraged
the parasites of insect pests, or other animals which prey upon them, for their
control. In the meantime the physiology of each kind of parasite can be studied
with the special object of determining its physiological relationship to its host,
on the one hand, and to the other parasites present in its environment inside that
host, on the other. It is this kind of knowledge of the nematode that is needed.
It can best be obtained by study of the nematode as a particular organism in
relation to its particular environment, the special problems of the economic worker
being kept in mind while this biological study is going on.
This kind of study, the natural one for the biologist, has already taught us
that the nematode is peculiar enough among living things, morphologically at
any rate, to require a separate phylum in zoological classification. The parasito-
logist is not primarily interested in such interesting biological features as the
nematode's lack of segmentation, nor its peculiar body cavity, which may or
may not be a coelom. He need not seek the reason for the remarkable uniformity
of structure among the nematodes, although it will be worth his while to note
that the reason for it cannot be that which is commonly given to explain the
similar lack of structural variation among the birds, namely, the narrow limits
of possible variation imposed by life in the air. The nematodes are, unlike the
birds, remarkably successful in a variety of environments. They live as well
in the soil, as on a sewage farm or in a vinegar barrel, and succeed as parasites
not only in the gut, but also in a great variety of tissues, and even in such
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