II. INTRODUCTION. 13 



of course all the indispensable conditions of life exist, and consequently we cannot 

 wonder at their being infested with other living beings adapted to their parasitic 

 position. Nevertheless, although the conditions of life are necessarily ever present 

 in living beings, yet these frequently do not contain parasites. There are many 

 circumstances besides those essential to life in general, which influence the exist- 

 ence or non-existence of such forms. One of the most important of these circum- 

 stances is the convenience or ease of access, or of entrance to the living body 

 infested. 



Within the living, closed, organic cell parasites very rarely if ever exist, because 

 it is liquid matter only which can endosrnose through cell-membrane, and, there- 

 fore, solid germs cannot enter, 1 and hence the unfrequency of true entozoa in 

 vegetables. Entozoa may and do penetrate through living tissues, but it is entirely 

 by the mechanical process of boring. 



The intestinal canal of animals is most frequently infested by entoparasites 

 on account of the ease with which their germs enter with the food. 



Aquaticanimals 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 rela- 

 tively 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. 2 



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 hemi- 

 pterous insects, aphides, etc., because such food is necessarily free from parasites or 

 their germs. Entozoa themselves, on this account, are not infested. 



On the other hand, if the liquid food be open to the air, parasitic germs may be 

 readily introduced into the animal, as in the case of the common house-fly, which 

 often contains myriads of a species of Bodo. 



Food swallowed in large morsels favors the introduction of attached parasites; 

 hence these are frequently found in reptiles, and even in birds, which are. among 

 the vertebrata, of the highest organic activity. 



Animals of feeble organic activity using solid food, which is very slowly digested, 



1 In some experiments upon the endosmosis of solid matter through organic cell-membrane, I found that 

 particles of carmine, diffused in water, which I estimated to measure about the 52,000th of an inch, would 

 no more penetrate the cell-membrane than the largest masses. 



a The inhabitants of the United States appear to be less infested with entozoa than those of any other 

 part of the world. This probably arises to a great extent from the more nutritious character of their 

 food j even the poorest laborer being daily supplied with abundance of wholesome flesh, producing a tend- 

 ency to high organic activity, which is unfavorable to parasitic development. 

 3 



