130 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



pendent for existence upon their hosts, are called obligatory parasites; 

 V those which are not, such as the infusoria usually found in the stomach 

 of herbivorous animals, are facultative parasites. Faculative parasites 

 often feed upon organic material provided by the host, and not upon 

 the host itself; but they are capable of living indefinitely apart from 

 the host. 



If an organism is attached to a host, and neither harms nor benefits 

 it, such an organism and its host are said to be commensals. For 

 example, the spirochsetes found about the teeth of many persons are 

 usually harmless; they are commensals of their host. If the host of an 

 obligatory parasite dies, the parasite may .perish also. Consequently, 

 it is contrary to the interest of such a parasite to destroy its host; yet 

 parasites often do harm their hosts. The harm done by a parasite to 

 its host expresses itself in derangements in the physiology of the latter 

 which are known as disease. The pathogenic protozoa may injure 

 their hosts in at least three ways: They may feed upon, and destroy 

 cells; they may produce poisonous toxins; and their presence may do 

 damage by mechanically obstructing some of the functions of its 

 host. All three of these ways are well exemplified by the action of the 

 malarial parasite in man (page 832). 



Discussion os the Classification* 



The following grouping of the Protozoa gives a general idea of the 

 position, in zoological sequence, pf the individual parasites which are 

 spoken of in the subsequent pages. The Protozoa are here grouped 

 into four classes: the rhizopoda, the flagellata, the sporozoa, and 

 the infusoria; and these classes are divided directly into genera. This 

 is by no means 9, complete classification of the protozoan families, for 

 there are many orders, families and genera which are unmentioned 

 because they are parasitic neither in man nor in animals. 



The form of a protozoon may vary greatly at different stages of its 

 development; for example, the adult herpetomonas is an active organism 

 moving by means of a flagellum, quite unlike its spherical form which 

 is, without a flagellum. Consequently, the whole life history of a proto- 

 zoon must be known before it can be classified with absolute certainty. 

 The whole of the life history is fcnown for only a fewprotozo'a; and, 



•(See p.' 13.) 



