no INFECTIOUS AND PARASITIC DISEASES. 



parasites are remarkable for the complicated and un- 

 usual manner by which they attain maturity; indeed, 

 it is owing to this unusual evolution that they produce 

 pathological states. Nearly all require for their com- 

 plete growth at least two hosts, man and some other 

 creature, in each of which complementary stages of 

 development take place. They may also have a stage 

 of growth in water or in the ground. Taken together, 

 the progressive development of a parasite from egg to 

 adult is called its cycle. Parasites which require two 

 hosts to complete their cycle may pass these in hosts 

 which are biologically allied, as for example, the beef 

 and pork tape-worms, whose hosts other than man are 

 respectively implied in their names. Often, however, 

 the dissimilitude in hosts is truly remarkable, and 

 causes us to marvel at the ingenuity which unraveled 

 their cycles. Thus, the parasites of malaria, yellow 

 fever, and filariasis, have each a stage of development 

 in particular mosquitoes; the guinea- worm, in a water- 

 flea; the schistoma haematobium, in a special species 

 of snail. The examples given above illustrate a curious 

 phenomenon in regard to parasites and hosts to which 

 there are few exceptions, viz., that each parasite is 

 restricted to hosts which enable it to complete its cycle. 



Note: — For a fuller description than is given here of animal para- 

 sites, well illustrated, the general reader is referred to Tyson's "Prac- 

 tice of Medicine," 4th ed.; or numerous articles in Woods' " Ref. 

 Handbook of the Medical Sciences," last ed. 



