Whether dealing with the external or internal forms, the study of parasites of man and animals is practically one of boundless extent; and there is probably no department of knowledge, possessing an equal value in relation to the welfare of man and beast, that is so thoroughly misunderstood by those who are directly concerned in the appreciation of its revelations. This has arisen from a total misconception as to cause and effect. Most people, not excluding even the votaries of the healing art, following tradition, regard the internal parasites or entozoa as creatures either directly resulting from certain diseased conditions of their _hosts_ or as organisms which would not have existed if their _bearers_ had been perfectly healthy. Nothing can be more absurd. Such a conclusion is utterly at variance with all logical deduction from known facts. It is, however, quite on a par with multitudes of other popular delusions which, in spite of the advance of science, will probably never become wholly eradicated from the public mind. People who hold these notions either cannot or do not desire to reject a view which has for them a dominating power almost equal to that of any known religious dogma. In conversation I have repeatedly noticed this to be the case. These people are the victims of educated ignorance and they will never allow that parasites are natural developments, accomplishing ends or parts of the orderly mystery which reigns everywhere. Some of then still cling to the creed that the presence of parasites, of internal ones at least, betokens evidence of Divine disfavor; and their minds are troubled with all sorts of distressing and childish conceptions. In the present age one would have thought that such ridiculous ideas could not be seriously maintained; but instead of being relegated to the limbo of similar “old wives’ fables” they dominate the opinions of thousands of our so-called educated people. The genuine searcher after truth does not need to be told that all preconceptions of this order hopelessly obscure the mental vision. They operate to render a just and adequate understanding of the science of helminthology impossible. The biologist may say what he lists, but he knows perfectly well that the superstitious mind will continue to ignore the precious and elevating results of scientific research, and that it will perseveringly continue to persuade itself that internal worms, parasites, and entozoa, of whatever kind, belong to the category of “plagues” liable to be distributed as special punishments for human wrong-doing.