76 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



to organisms belonging to the animal kingdom, parasites may be either 

 animal or vegetable; bacteria and fungi, which live at the expense of 

 other living beings, are parasites just as the disease-producing protozoa, 

 and the biting insects which transmit them, are parasites. 



Most parasites are simple organisms, low in the scale of life. They 

 nourish themselves without exertion, at the expense of their hosts, and, 

 as might be expected, their unemployed organs, such as the sensory, 

 locomotory and seizing appendages, through which food is usually ob- 

 tained, gradually disappear; degeneration always occurs in an organism 

 which assumes a parasitic mode of life. 



Organisms, such as the malarial parasite, which are wholly depen- 

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

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

 of herbivorous animals, are facultative parasites. Facultative parasites 

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

 host itself; they are then said to be saprophytic. 



If a parasite is attached to a host, and neither harms nor benefits it, 

 the parasite and host are said to be commensals. For example, the spiro- 

 chaetes 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 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 recognized by the 

 disease caused by*it. The pathogenic protozoa do harm to their hosts in 

 three main ways: They may feed upon, and destroy cells; they may 

 produce poisonous toxins; and their presence may do damage by me- 

 chanically obstructing some of the functions of its host. All three of 

 these ways are well exemplified by the action of the malarial parasite 

 on man (p. 685). 



DISCUSSION OF THE CLASSIFICATION.* 



This grouping of the protozoa gives a general idea of the position, in 

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

 the following pages. It simply divides the Protozoa into four classes: 

 the RHIZOPODA, the FLAGELLATA, the SPOROZOA, and the INFUSORIA; and 

 subdivides these classes directly into genera. This is by no means a 



* (See page 10.) 



