True parasites are beings entirely dependent on their neighbors for support; unable to provide for themselves, they are fed wholly at the expense of others. It is generally believed that they are an exceptional class of organisms, constituting a group by themselves, and knowing nothing of the world outside the organ which shelters them. This is an error. Representatives of all the principal divisions of the animal kingdom below the vertebrate are found pursuing this mode of life. There are few parasites that are not wanderers at some period of their lives; and it is not uncommon to find some which live alternately as noblemen and as beggars. Many are paupers only during infancy, or at the approach of adult age, living at other times a comparatively free and independent life. Nor are all the members of a species necessarily parasitic; sometimes it is only the female that takes the relation of a dependent, the male continuing his nomad life. Again, there are cases where, the female being provided for, the male relies on her for support, and thus the charitable animal which comes to her help is laid under contribution by the whole family.
Parasites present an extraordinary variety of forms, and differ very widely in size and appearance, these differences being often remarkable between the sexes of the same species. The male of the urubu of Brazil has the usual form of a round long worm, while the female resembles more than any thing else a ball of cotton, not having the slightest analogy with the other worms of the order. As to the enormous proportions parasites may attain, Boerhaave mentions a bothriocephalus 300 ells[1] in length; and, at the Academy of Copenhagen, it was reported that a solitary tapeworm (Tœnia solium) had been found 800 ells long. Parasites are found in every region of the globe, but, like other animals, they observe the laws of geographical distribution. Some, like the leeches, take their food, and then detach themselves until the demand for food returns, never becoming identified for any length of time with their host. Others, like the lernæans, commence their parasitic existence when approaching maturity, and thereafter are permanent dependents; others, again, like the ichneumons, begin life as parasites, and on reaching maturity assume and maintain an independent existence; while still others, like the tænia, are parasitic from first to last, although changing their abode at a certain stage of development.
All animals, man included, have their parasites, which usually come from without, those entering the body being generally introduced with the food or drink. No organ is exempt from their incursions, as they have been found in the brain, the ear, the eye, the heart, the blood, the lungs, the spinal cord, the nerves, the muscles, and even the bones. Cysticerci have been seen in nearly all these situations, and worms of various kinds are common in the cavities of the body, as well as in many of the solid organs, such as the muscles, liver, and
- ↑ The Flemish ell is probably meant: this is 27 inches long.
