EDMONDSON — PROTOZOA OF IOWA. 85 



The macroniicleus usually begins to break up just as the organ- 

 isms are about to separate, the period of nuclear reconstruction 

 extending over many days. After a wave of conjugation had 

 passed, on one occasion, I observed it to require from eighteen to 

 twenty-two days for a complete reorganization of the nuclear 

 elements. During this time no cell division takes place, the pro- 

 cess of conjugation, therefore, acting as a check upon rather than 

 hastening reproduction. 



Of what value then is conjugation? It is 1)elieved that as a 

 result of activity the cell becomes physiologically exhausted, some 

 elements vital to its existence being lost which, if not renewed, 

 will result in death to the organism. Conjugation is a means of 

 restoring those elements lost through natural activity of the pro- 

 toplasm. Calkins, however, after recent experimental work, 

 concludes that conjugation is a means of last resort and in a state 

 of nature probably seldom occurs. 



In the natural habitat of Paramirciutn, where conditions are 

 changing and new elements are ever being introduced into the 

 water from air and soil, probably there is little need for such 

 phenomena as conjugation. The laboratory infusion, however, 

 does not offer these changing conditions and conjugation must 

 frequently be resorted to. 



Usually, in infusions, these periods of conjugation occur after 

 more or less regular intervals. By the introduction into the 

 infusion of certain elements, such as animal and vegetable 

 extracts, salts, etc.. Calkins was able to revitalize Paranicpcinvi, 

 when periods of phj'sical depression occurred, for 742 generations. 



Paraiiufciinii caiidatiiin is abundantly supplied with trichocysts, 

 probably defensive weapons in this species, which are coiled just 

 beneath the cuticular suface when not in use. Stimuli of various 

 kinds will cause these thread-like structures to shoot out far 

 beyond the cilia, from all points of the periphery, until the body 

 fairly bristles with them. Weak acid solutions of various kinds 

 will cause them to be extended; often, however, if the solution be 

 very weak, the trichocysts will appear only at the two extrem- 

 ities, probably indicating that these regions are the more sensitive. 



Reproduction takes place by tran.sver.se division. 



Found everwhere in stagnant water and infusions. 



