xii INTRODUCTION 



of procuring food, take up their abode temporarily or perma- 

 nently, on or within other living organisms." This definition will 

 exclude predaceous animals (Raubtiere), which capture their 

 prey alive, and usually kill it outright for purposes of food. 



Classes of parasites. Other than the two general classes, 

 Ectoparasites (external parasites), and Entoparasites (internal 

 parasites), all parasites may be placed in one or another of the 

 following divisions, according to the time spent in or upon the 

 host. Facultative parasites have the power of changing from one 

 host to another of a different species, e. g. the cat and dog flea, 

 Ctenocephalus canis, which may be found upon the cat, the 

 dog, the rat and man; the rat flea, Ceratophyllus fasciatus, upon 

 the rat and upon man; the wood tick, Dermacentor variabilis, 

 may be found on nearly all species of domesticated animals and 

 man. Obligatory parasites are restricted to one species of host, 

 e. g. the biting bird lice (Mallophaga), which perish sooner or 

 later if transferred to another species of host. Intermittent par- 

 asites are such as prey upon the host for food, as in the female 

 horseflies (Tabanidae) attacking horses and cattle for the pur- 

 pose of sucking blood and leaving the host after a meal. Tran- 

 sitory parasites are such as pass part of the life history at the 

 expense of the host, for example, the botflies (CEstridae) which 

 pass the larval period of development within the body of the 

 host, while the adult botflies are free living and do not attack 

 other animals for food. 



Effect of parasitism on the host. That an animal is para- 

 sitized does not necessarily involve it in death, not even in great 

 inconvenience, although the parasite is actually living at its 

 expense. The presence of a few bots in the stomach of a horse 

 may not affect that animal in the least, nor would the presence 

 of a few lice on the body of an animal, but with the multiplica- 

 tion of these parasites there will be increased inconvenience to 

 both hosts. The presence of a few maggots in the fleshy part of 

 a sheep's leg might cause little damage, but let these be in the 

 nasal sinuses or at the base of the brain, and the gravity of the 



