RELATION OF PARASITIC TO FREE LIFE. 3 



and, in my opinion, also for frogs, snakes, and spiders, 1 which have 

 been stated by many authors to have existed for years in the human 

 alimentary canal, although it is perfectly certain that animals of this 

 kind cannot endure the damp heat of the body of a mammal for 

 more than six hours. 2 



This occasional parasitism sufficiently points out what has just 

 been maintained from another point of view, that no broad line of 

 demarcation can be drawn between parasites and free-living animals. 



It is not, however, in such instances alone that the transition 

 between the free and parasitic modes of existence is found. Many 

 animals (such as the leech) are only parasites so long as they obtain 

 their nourishment at the expense of larger and more powerful 

 creatures, becoming simply carnivorous when they prey upon other 

 animals of their own size or smaller. A parasite is, in all cases, 

 smaller and weaker than the animal on which it feeds. Being in- 

 capable, therefore, of overpowering it, the parasite contents itself with 

 plundering its host and drawing nourishment from its juices and flesh. 



Thus the parasitic and free modes of existence are related to 

 each other in two distinct ways, both of which are connected with 

 peculiarities of parasitism itself, one of these links being the nature 

 of the food, the other the relation of the parasite to the animal which 

 supplies the nourishment. Eeflecting upon the significance, already 

 pointed out, which the size and equipment of the parasites have with 

 regard to their mode of life, it is not surprising that the various groups 

 of the animal kingdom do not all furnish equal contingents to the 

 army of parasites. Among the Vertebrata, for instance the majority 

 of which are strong and of large size there are very few parasitic 

 forms ; while, on the other hand, among the comparatively small and 

 feeble Arthropoda and worms there are entire families, all, or nearly 

 all, the members of which lead a parasitic life. In fact, it may safely 

 be asserted that these two groups contribute more parasitic forms 

 than all the other divisions of the animal kingdom taken together. 



1 In these cases, also, the microscope serves to dispel the illusion ; for the contents 

 of the intestines of these pseudoparasites will contain substances that could not possibly 

 have been obtained in the body of their host. In estimating the origin of various objects 

 asserted to have been evacuated by a patient it is impossible to be too careful. In such 

 cases there is frequently an attempt to deceive the medical man, but more usually some 

 error has been introduced through a variety of circumstances. If, for instance, everything 

 that is found mixed with the faeces be, without further investigation, set down as having 

 come from the body of the patient, then the famous helminthologist, Dr. Bremser, must 

 have evacuated, as he humorously relates, a pair of snuffers ; for they were certainly 

 found in the bed-pan at a time when he was slightly indisposed, without any one having 

 placed them there. 



2 Berthold, "Ueber lebende Amphibien im lebenden Korper," Mutter's Archivf. Anat. 

 u. Physiol., p. 430, 1849. 



