INTRODUCTION. 



Tbe recent excellent works by Diijardin,' Diesing," and Eoliin,^ 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 entoph^ta, 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 posi- 

 tion, 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 entophj^ta, 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 ento- 

 phyta are produced from germs derived from parents, and having a cyclical deve- 

 loiiment. 



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. If, however, an entozoon preserved 

 the same form throughout its migrations, the difficulty just mentioned would be 



* Histoire Naturelle des Helmintlies, Paris, 1845. 

 ° Systema Helminthum, Viiulobonaj, 18.50. 



^ Des Vegetaux qui croisscnt sur rhomiue ct sur Ics animaux vivants, Paris, 1817. 

 o 



