OF THE ALIMENTARY TRACT 13 



form and number. In those which multiply within 

 the cysts, the nucleus divides, by mitosis, and each 

 daughter nucleus continues the division until the 

 full number for the species is produced. 



When the process of intracystic reproduction is in- 

 complete the nuclei are usually large and hyperchro- 

 matic, when complete they are characteristic in size, 

 shape and appearance. 



Cysts in their natural state appear as glassy or 

 hyaline bodies. 



LIFE HISTORY 



Protozoa either spend their lives as free-living 

 animals in suitable environment, usually in water, 

 or they live in connection with other animals. Ac- 

 cording to the nature of this association they may be 

 commensals, which are dependent upon their asso- 

 ciation with their hosts but cause them no damage. 

 Others live in symbiosis, where both of the parties 

 to the association derive some benefit from it. Still 

 others are parasitic upon their hosts, that is they 

 cause some injury by utilization of materials needed 

 by the host or by producing substances harmful to 

 the host tissues. In this strict sense the protozoa 

 here under consideration are either commensals or 

 parasites and the debate has been as to which class 

 the several kinds belong. 



It has come to be the popular conception that "a 

 living organism which, for the purpose of procuring 

 food, takes up its abode temporarily or permanently, 



