INTRODUCTORY 3 



are then said to be specific to that class. Thus Tccnia 

 solium and T. saginata, in their adult condition, are specific 

 to man. 



Some parasites are common to several classes of hosts ; 

 these hosts, as a rule, are related fairly closely, but not 

 invariably so. Trichina spiralis is common to man, pigs 

 and rats, as well as to many other animals. 



Parasites that are very abundant in some hosts are 

 found occasionally in other animals. When this occur- 

 rence is exceptional the parasites are said to be incidental 

 or accidental. Thus Fasciola hepatica, so common in 

 sheep and cattle, may be an incidental parasite of man. 



Effects of Parasites on their Hosts. The diseases due to 

 animal parasites may be the direct results of injuries 

 mechanical effects caused by the parasites, their eggs or 

 embryos ; or they may be caused by a poison toxin- 

 elaborated by the parasites themselves ; or they may cause 

 conditions which favour the introduction of other para- 

 sites, animal or vegetable, which set up definite diseases. 

 Frequently both effects are produced and play a part in 

 the special characters of the disease which results from 

 their presence. 



Injurious Non-parasitic Metazoa. Metazoa not necess- 

 arily parasites in a strict sense are, in some cases, injurious 

 to man. Many of the insects attack man, and by their 

 bites or stings produce effects, either trivial or merely 

 local, as in the case of mosquitoes or sandflies, or more 

 severe, as in some of the Hymenoptera, such as wasps 

 and hornets. Ticks, spiders and scorpions may similarly 

 cause local lesions or general poisoning when they attack 

 men. 



Metazoa as Carriers of Parasites. Some of the Arthro- 

 poda, such as mosquitoes, tsetse-flies and ticks, are the 

 active and essential agents in the transmission from man 

 to man of dangerous parasitic protozoa, as those of 

 malaria, relapsing fever and sleeping sickness ; or meta- 

 zoa ; such as those causing elephantiasis and other forms 

 of filariasis. 



