The recent excellent works by Dujardin, Diesing, and Kobin upon animal and vegetable parasites of living animals, render another systematic record of the labors in this field almost superfluous; and the object of the present memoir is simply to give the result of a series of observations, commenced several years ago, upon associated entozoa and entophyta, constituting a flora and fauna within animals.
The existence of entozoa, or of animals living within other species has, from the most remote time, attracted attention, on account of the peculiarity of their position, the unpleasant ideas associated with them, the sufferings they frequently induce, and the difficulty of explaining their mode of origin.
The entozoa have always constituted the strongest support of the doctrine of equivocal or spontaneous generation, one which has found distinguished disciples even to the present time; but since the days when barnacles were supposed to originate from the foam of the ocean, and ducks and geese to be developed from barnacles, this belief has been so weakened by the accumulation of facts, undenied and undeniable by the supporters of the doctrine, that it bids fair soon to be little more than an echo of the past.
The existence of vegetable parasites within animals, or of entophyta, from their minuteness, remained unknown, until the microscope of Leeuwenhoek detected the algoid filaments of the human mouth; but it is only within a comparatively recent period that any large number of distinct parasitic plants has been discovered.
The very great majority of modern observations indicate that entozoa and entophyta are produced from germs derived from parents, and having a cyclical development.
A great difficulty in determining the course of this development, particularly with entozoa, is, that their various stages of existence are passed under totally different circumstances; sometimes within one organ and then another of the same animal; sometimes in several animals; and, at other times, even quite external to and independent of the animals they infest.