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PLASMODROMATA AND SARCODINA 



SUBDIVISION A. PROTOZOAN PARASITES. 



SUBKINGDOM I. PROTOZOA Goldfuss, 1S17. 



Definition. — -Protozoa are unicellular animals, solitary or united 

 into colonies, free-living or parasitic, with asexual reproduction 

 (schizogony) by binary fission, budding or fragmentation and sexual 

 reproduction (sporogony), or merely rejuvenescence by conjugation. 



Remarks. — ^The border-line between unicellular animals and uni- 

 cellular plants is very vague, and hence at present it is uncertain 

 whether some forms should be classed with the bacteria and con- 

 sidered to be plants, or with the protozoa and considered to be 

 animals. Therefore it is not unusual to call both ' protists.' 



Phylogenesis. — ^There can be little doubt that the primitive form 

 of protozoon must have been an animal with sonie of the charac- 

 teristics of the amoeba, and would most likely be free-living, most 

 probably in water. This simple form, taken into the alimentary 

 canal of higher animals, possibly benefited by the ease with which 

 food was obtained and by the protection afforded by the new 

 position, and thus became modified to suit its environment. The 

 most important modification would be some protection for the 

 earliest stages of its life-history, which would enable it to live in 

 the outside world until taken up by a suitable host, and associated 

 with this would be the necessity to produce large numbers of such 

 protected spores, as the chance of one finding a suitable host must 

 be relatively small. Such a type would be represented by the 

 Loeschia coli Loesch, which is a parasite of man. Such a parasite 

 may cause no harm to its host, which is fairly indifferent to its 

 existence. On the other hand, a parasite may not find sufficient 

 nutriment in the alimentary canal, and be compelled to seek better 

 food, and perhaps more protection, by entering the glands of 

 Lieberkiihn, or even the sub mucosa of the bowel. Such a parasite 

 might cause disease in its host, and would be illustrated by the 

 Loeschia histolytica Schaudinn of man. 



Such a process, however, took place not merely in man, but in 

 inany other kinds of animals, among which may be mentioned 

 insects, in the alimentary canal of many of which protozoan para- 

 sites are found. In these insects the sexual process occurs, and 

 therefore they are the definitive hosts, and may also be considered 

 the primary hosts. 



If these primary hosts become predatory, biting and sucking the 

 blood of other animals — e.g,, vertebrates — then, during this process, 

 they might pass the spores of their parasites into the blood of the 

 vertebrata, and if these are not killed off (for some animals are 

 repellent) by chemical substances, or destroyed by leucocytes, they 

 might develop in the blood of some vertebrate (called tolerant), 

 which thus becomes the secondary host. 



It is, of course, possible that some of these blood protozoa may 



