Animal Piirasilcs. yy 



by microorganisnis ai^prcciablc only with the aid of the niiero- 

 scopc. the protophytcs and protozoa, a condition spoken of as in- 

 fection wlien leading" to disease. Some of these micfobes well 

 merit the name parasites, since the animal body in which they 

 exist is the source of their nutrition, but the toxic properties of 

 the most of them are so conspicuous that we are likely to think 

 of these not so much as ])arasites, but as comparable to poison- 

 ous plants. 



A mmiber of forms of worms and arthropoda (zooparasites) 

 have gradually assumed the character of true parasites and actu- 

 ally live and subsist upon the body structures. ( )ccupation of the 

 body, either externally or internally, spoken of as i)n'asioii 

 (iiiz'adcrc, to penetrate), is apt to give rise to a varied group of 

 tissue alterations and disease processes (invasion diseases [iiifest- 

 ment] ). The individual in which such a zooparasite finds shelter is 

 to be looked upon as the host of the parasite. Some infest the host 

 only for a brief period as necessary for the acquirement of nutri- 

 tion (transiriif <^r tcutporary parasites) , or for the time essential 

 for the completion of one or more stages of their development, 

 among which groups respectively fleas and the larvfe of gadflies are 

 examples. Others are persistent and pass their entire existence as 

 parasites (habitual or pcniiaiicut parasites). The life of this 

 latter type ma\- be passed in some instances entirely in one host 

 (inono.rcnoiis parasites: ^bvos, one; ^evos, host), or may be dis- 

 tributed over several hosts (hetero.veiioiis parasites). 



Invasion of a host at times is merely the result of accident, 

 in which case the parasite is usually taken in with food: in other 

 instances this or that parasite may by its own independent move- 

 ments gain entrance into the body. 



Parasitism may he referred to two underlying conditions, 

 adaptability of the parasitic organism to the conditions of life af- 

 forded by the body of the host and the biological variability of the 

 former. Worms and arthropods which have assumed parasitic char- 

 acteristics undoubtedly at one time lived independently in nature. It 

 may be imagined that one of these low types of animals by acci- 

 dent entered the bodv of some higher animal, and, finding condi- 

 tions favorable for the acqiiiremcnt of food, remained and multi- 

 plied ; that the offspring, now native to the body of the host, 

 did not again seek a free state, but continued as parasites, being 

 from birth accustomed to the altered environments. Such sup- 

 position is justified by the fact that some parasites still show rudi- 



