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etal kingdoms. Its manifestations are so varied in their character 

 that the very term is difficult to define. We have the very begin 

 nings in mere association or commensalism, and every varia 

 tion from this beginning, through partial parasitism, to that most 

 complete form in which the parasite is confined to a single host 

 and is throughout life dependent upon it for existence. In whatever 

 phase it presents itself, however, the subject of parasitism is of 

 intense interest from the standpoints of morphology, biology, and 

 evolution, as in no class of animals is the effect of habit and en 

 vironment in modifying structure more markedly or graphically 

 demonstrated. It would be obviously impossible to treat even in 

 the most casual wav of the subject of parasitism in other classes 

 of animals. It suffices to say that parasites are found in nearly 

 all orders of animals, from the Protozoans to the Vertebrates ; 

 that the Helminths and Bacteria are essentially parasitic, and that 

 even the Mollusks, which are essentially non-parasitic, contain 

 some curious parasitic forms. The Siphonostomous Crustacea 

 (Epizoa) occupy the same parasitic place among aquatic animals 

 as the insects do among terrestrial animals ; yet I cannot treat here 

 with satisfaction of parasitism among Arthropods generally, but 

 must confine my remarks substantially to Hexapods. 



ANIMALS AFFECTED. 



VERTEBRATES. Among vertebrates, mammals and birds are 

 most extensively affected by insect parasites, reptiles and fishes 

 being comparatively exempt. 



Primates, including man, are very commonly affected by true 

 lice, and not infrequently, in southern climates, by certain bots, 

 notably Dermatobia noxialis. Flesh sores and the nasal passages, 

 especially when affected by catarrh, are infested by certain Dip 

 terous larva*, not normally parasitic, and in some cases, as in that 

 of our southern Screw Worm {Lucilia macellaria}, the injury 

 may become serious. Some of the Acarina are often exceedingly 

 annoying, chiefly in the larva state ; but with the exception of some 

 of the Sarcoptidae, like the itch and mange mites, they are not to 

 be looked upon as true parasites. Carnivora are affected by true 

 lice, by bot-flies, bird-lice (Mallophaga),and mites. They are, in 

 deed, very susceptible to the attacks of these parasites, and the 



