How Parasites Get In

The intestinal canal of animals is most frequently infested by entoparasites on account of the ease with which their germs enter with the food. Aquatic animals are more troubled by entozoa than those which are terrestrial, because the water affords a better medium of access than the air. Terrestrial animals, on the other hand, are more infested by ectoparasites because their covering of hair, wool, and feathers is more favorable to their protection and reproduction. A low degree of organic activity and slowly digestible food favor the development of entoparasites, and hence they are more frequent in the relatively sluggish herbivora than in the carnivora. Comparatively indigestible food, and such as contains but a small proportion of nutritive matter, from its long retention in the alimentary canal, favors the development of entozoic and entophytic germs more than that in which the contrary conditions prevail. Animals subsisting upon the endosmosed juices of the tissues of other animals, and of plants, are rarely infested by parasites, as in the case of the hemipterous insects, aphides, etc., because such food is necessarily free from parasites or their germs. Entozoa themselves, on this account, are not infested.

Leidy sets out the doors by which parasites enter a body: with food, through water, and under hair, wool and feathers.

Source: Joseph Leidy, A Flora and Fauna within Living Animals, Smithsonian Contributions to Knowledge (Washington: Smithsonian Institution, 1853), Introduction, p. 13. Text: scan or ebook. Public domain in the US. Text note: Copied from the scan's OCR and checked word for word against the page image. Line-break hyphens were rejoined and one footnote mark was removed. Pairs with: Cups: Page or Two of Cups, or Major Arcana: Temperance A note from 2026: Several claims here have not held up. Sap-feeders such as aphids do carry parasites and symbionts, parasites can have parasites of their own, and plant-eaters are not reliably more parasitized than meat-eaters.

Public domain in the United States.