PROTOZOA 133 



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. When the host of an 

 obligatory parasite dies, the parasite often perishes 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 is the disease which that parasite causes. Disease is recognized by 

 symptoms. The nature of the symptoms depends directly upon the 

 nature of the harm done by the parasite. The symptoms are the result 

 of interference by the parasite with tissues, or the functions of tissues, 

 in the host. 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 892). 



DISCUSSION OF THE C L AS SIP J CATION* 



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

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

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

 in four classes: the RHIZOPODA, the FLAGELLATA, the SPOROZOA, and 

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

 is by no means a complete classification of the protozoan families. 

 Many orders, families and genera are unmentioned because they are 

 parasitic neither in man nor in animals; and of the organisms mentioned, 

 only those which are constantly causes of disease are described. 



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 known for only a few protozoa;. and, 



(See p. 13.) 



