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apparently unsuitable tissues as tendon and ligamentum nuchae. They seem,
indeed, like many other parasites, to favour such tissues as liver and muscle, in
which glycogen is abundant, a point which will be discussed below. They live
as parasites in both warm and cold blooded animals, in vertebrates as well as in
invertebrates, and are parasites also in plants. Yet there is little essential
difference of structure between the various species living in these every different
environments. Many nematodes have performed the feat, without any morpho-
logical penalty, of living part of their lives as parasites inside a host, and part of
it in the outer world, requiring no metamorphosis as a condition of this kind of
parasitism; the parasitic larvae of such species are not essentially different in
structure from the free-living ones preceding them in the life history.
Even these few general facts are enough to show that the nematodes are
extremely adaptable. Their organs, although they show so little structural
variation, can successfully perform their functions in widely differing environments.
Few animals excel the nematode in its capacity of resisting adverse conditions.
We may expect to find, therefore, that their physiological needs either are not
exacting or can be supplied by perhaps more primitive methods than those to
which more highly organised animals are confined. If, for example, they require
little oxygen, a fact which has been demonstrated by McCoy (1930) and others,
we may find that they also show peculiar methods of respiration, a knowledge
of which might suggest to us means of control. We may find among them a
capacity to feed on a great variety of substances ; and a system of excretion of
a primitive kind. Their muscles, we know, are morphologically peculiar and
the study of them may give us useful information. Most important of all,
perhaps, from the point of view of the economic worker, is their peculiar cuticle,
a structure unique in the animal kingdom. We do not know even the chemical
composition of this; certainly we do not know its function, nor anything about
its permeability. It seems especially important, for example, to learn some-
thing about the permeability of this cuticle to the drugs with which we try to
kill its possessor.
Our lack of knowledge about all these points constitutes one of the most
remarkable gaps in comparative physiology. A brief review of such knowledge
as we have may help to indicate some lines of investigation which, although they
should be pursued from the purely biological point of view, may yet be especially
interesting to the parasitologist, or may be especially the kind of work he can do.
Because many parasitic nematodes have non-parasitic larvae by which their
hosts are infected, the problems raised by these free-living larvae must also be
considered by the parasitologist. It will be convenient, therefore, to consider
the eggs and the free-living larvae first and the parasitic larvae and the mature
worms in a separate section.
It should not be forgotten, however, that the same kind of organism is being
considered in both sections. The non-parasitic larvae often live under conditions