VETERINARY MEDICINE. 



PARASITES AND PARASITISM. 



Mutualism, Commensalism, Parasitism, Symbiosis, Rapacious enemies, 

 venomous. Phytoparasites. Zooparasites. Microbes : protozoa, bacteria. 

 Ectoparasites, vegetable, animal. Entoparasites, vegetable, animal. Com- 

 pulsory parasites. Facultative parasites. Temporary parasites. Intermit- 

 tent, remittent, erratic. Monoxenous. Heteroxenous. Varying gravity 

 of parasitisms. Conditions which increase gravity. 



When two animals or plants, or an animal and plant form an 

 interdependent existence or association the relation is explained 

 by one of three designations, mutualism, commensalism or 

 parasitism. 



Mutualism is where two live together and are mutually de- 

 pendent on each other, but the union is beneficial to both. Thus 

 the acari that live in the feathers of birds and feed on the 

 epidermic debris or dandruff are actually beneficial to their hosts 

 and are strictly speaking not parasites. 



Commensalism is where two live together, and the union is 

 beneficial to one of the parties without proving injurious to the 

 other. Myriads of infusoria live in the stomachs and intestines 

 of ruminants but cause no appreciable injury, if indeed they are 

 not beneficial in disintegrating the iugesta. 



Parasitism differs from both in that one of the two draws its 

 subsistence from the other to its appreciable injury. 



The common term symbiosis (living together) may be applied 

 to all three, but in mutualism the symbiosis is beneficial to both, 

 while in commensalism the symbiosis is beneficial to one without 

 harming the other, and in parasitism the symbiosis is antagonistic 

 and injurious to the host. 



Another class which is closely allied to parasites is more 

 purely rapacious. Beside the carnivorous mammals these em- 

 brace a number of smaller animals like leeches, mosquitoes, flies, 

 bed bugs, etc. 



