44 PROTOZOA 



If Stentor is cut into two pieces, any part containing a portion 

 of the nucleus will regenerate readily. 



Endomixis. — There is some controversy over the significance of 

 the complete nuclear reorganization without cell fusion first de- 

 scribed by Woodruff as occurring periodically in pedigreed races of 

 Paramecium. At regular intervals of about 30 days, in Parameciurn 

 aurelia (sixty in P. caudatum) the old jnacronucleus gives rise to 

 buds or fragments which are absorbed in the cytoplasm. Each of 

 the micronuclei divides twice, which forms new products of both 

 micro- and macronuclei. 



It is the belief of Calkins, from his investigations on Uroleptus, 

 that " endomixis " is a satisfactory substitute for the fusion of 

 different nuclei at conjugation. He holds that continued vitality 

 is possible when either process furnishes the necessary reorganization 

 of nuclear elements. Endomixis does not seem to be essential to 

 all ciliates. It has been interpreted as parthenogenesis. 



Distribution of the Protozoa. — Even the most barren soils con- 

 tain Protozoa, the same species being found in tropical, temperate, 

 and arctic soils. The maximum numbers of soil Protozoa are found 

 at a depth of 4 to 5 inches, but some species seem able to live under 

 anaerobic conditions. Certain Protozoa thrive as internal parasites, 

 and live in the blood or internal organs of other animals. 



The majority of the Protozoa, however, are aquatic. They are 

 found from the deepest seas (5,000 fathoms) to a point 10,000 feet 

 above sea level, Juday reported a fresh water anaerobic ciliate 

 found in the centrifuged plankton of Lake Mendota. It appeared 

 in water containing a minimum amount of oxygen. The maximum 

 number (95,250) appeared in a litre of water from a stratum having 

 no oxygen. 



Pack reported 1 ciliata, 9 algae, 5 bacteria, i crustacean and 2 

 fly larvae in the water of Great Salt Lake which has 23 per cent 

 salinity. Dilution of the medium caused a shortening of the cirri 

 of the ciliates with increased size and activity. 



Fossil Relatives and Relationship to Other Phyla. — The Fora- 

 minifera are of great geological importance, and are common as 

 fossils from the Silurian rocks down to the present time. Today 

 they are found in calcareous ooze, and are building beds of chalk 

 and nummulithic limestone. 



The siliceous skeletons of the Radiolaria are found in slate and 

 deep sea ooze. They aid in the formation of flint. (See p. 52.) 



