Excerpt from Of Veterinary Medicine, Vol. 5: Parasites, Parasitisms, Etc
Mutualism, Commensalism, Parasitism, Symbiosis, Rapacious enemies, venomous. Phytoparasites. Zooparasites. Microbes; protozoa, bacteria. Ectoparasites, vegetable, animal. Entoparasites, vegetable, animal. Compulsory parasites. Facultative parasites. Temporary parasites. Intermittent, remittent, erratic. Monoxenous. Heteroxenous. Varying gravity of parasitisms. Conditions which increase gravity. Infection - bearing parasites.
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 dependent 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 ingesta.
Parasitism differs from both in that one of the two draws its subsistence from the other to the appreciable injury of the latter.
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 symbiosis is antagonistic and injurious to the host.
Another class which is closely allied to parasites is more purely rapacious. Besides the carnivorous vertebrates these embrace a number of smaller animals like leeches, mosquitoes, files, bed bugs, etc.
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