NUTRITION 



no other section in which we can follow, step by step, 

 the'course of the degeneration which is caused, and show 

 clearly the mechanical nature of the process. Hence 

 the science of parasites-^parasitology — is. one of the 

 soundest supports of the theory of descent, and provides 

 an abundance of the most striking proofs of the much- 

 contested inheritance of acquired characteristics. 



Among the unicellular organisms, the bacteria are 

 the most conspicuous instances of manifold adaptation 

 to parasitic habits. As we count these unnucleated 

 protozoa among the oldest and simplest organisms, and 

 trace them directly by metasitism to the plasmodomous 

 chromacea, it is very probable that they turned to para- 

 sitism very early in the history of life. Even a part of 

 the monera (in which group we must place the bacteria 

 on account of their lack of a nucleus) found it conven- 

 ient and advantageous to prey on other protists and as- 

 similate their plasm directly, instead of going through 

 the laborious process of carbon assimilation themselves 

 in the hereditary fashion. This is also true of the large 

 class of the sporozoa or fungilla (gregarinm, coccidia, etc.), 

 real nucleated cells, which have adapted themselves in 

 various ways to parasitic habits. Many of them live in 

 the rectum, the coelum, or other organs of the higher 

 animals (the gregarinae, especially in the articulates); 

 others in the tissues (for instance, the sarcosporidia in 

 the muscles of mammals, the coccidia and myxosporidia 

 in the liver of vertebrates). A good many of them are 

 "cell-parasites," and live inside the cells of other ani- 

 mals, which they destroy; such are the hoemosporidia, 

 which destroy the blood-cells in man,a,nd so cause inter- 

 mittent fever. 



Among the multicellular metaphyta it is particularly 

 the fungi that have taken to parasitism in various ways. 

 Many of them are, as is known, the most dangerous 

 enemies of the higher animals and plants. The various 



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