150 BIOLOGY FOR BEGINNERS 



has a fixed shape; cilia for locomotion and food-getting; a definite 

 mouth and gullet, and definite regions for excretion. This increase 

 in adaptation of structure to function is called specialization, or 

 division of labor, and is the mark of higher development in any 

 plant or animal. 



Reproduction. In paramcecium this function is more highly 

 developed than in amoeba and consists of two processes, fission 

 and conjugation. Fission takes place, preceded, as usual, by the 

 division of the nucleus, and two new individuals are produced, 

 much as in amoeba, but in a more definite manner. This process 

 can go on for only about 150 times, when the vitality seems to be 

 reduced and conjugation takes place. 



In conjugation, two paramcecia unite by joining the region near 

 the " mouth " cavity, and their cell wall becomes thin at the point 

 of union. Complicated divisions take place in the nucleus of each 

 and finally a stage is reached where there are two parts to each 

 nucleus, one of which is stationary and the other not. The two 

 movable nuclei now exchange places, passing through the pro- 

 toplasm of the cells and finally unite with the stationary nucleus of 

 the opposite individual. After Jthis exchange and union of nuclei 

 the paramcecia separate again. There has been no gain in numbers 

 but the vitality of the protoplasm has been increased so that re- 

 production by fission can go on again. 



This conjugation does not make more individuals as true 

 reproduction does, but it enables both participants to repro- 

 duce by fission and is the first step toward fertilization in ani- 

 mals, which, as in plants, is the union of two different cells from 

 two individuals. 



Parasitic Protozoans. Some protozoans are parasitic, attack- 

 ing other animals and producing serious diseases, much as do the 

 bacteria. They are often classified with the latter as " disease 

 germs " or " microbes." If we realize that these terms include 

 both one-celled parasitic plants (bacteria) and one-celled parasitic 

 animals (protozoa) then their use is correct. 



Some diseases caused by protozoan parasites are in the following 

 list. The way in which they are transmitted will be more fully 

 discussed under insects (Chapter XXV). 



