THE SIMPLEST ANIMALS 



303 



FIG. 89. Division or re- 



SSftar'fe 



Paramecium. n, nucleus; 



^ c } uole ' (From 



among one-celled animals. It is simple cell-division, but 

 the two new cells separate, instead of remaining together as 

 in the tissues of higher plants and ani- 

 mals. This is a^^ua]^reprQ.dM(Man. 

 Conjugation. Under certain con- 

 ditions, not yet entirely understood, 

 paramecia reach a state when they are 

 unable to continue to divide. Two 

 such individuals come into contact, 

 and through their delicate cell-walls 

 some of the nucleus of each one passes 

 over to join the nucleus of the other^. 

 the result being in each individual a 



nfiBUlUcleUS, half of which has come 



from another paramecium. Then 



the two animals SWim away inde- 



pendently, each soon divides, and 



their offspring may continue to divide 



for a long series of generations before this exchange of 

 nuclear substance again takes place. 

 This process is known as conjugation 

 and in essentials is similar to conjuga- 

 tion in molds ( 244) and Spirogyra 

 ( 239), and is believed to have the 

 same effect on heredity as has fertili- 

 zation in higher animals (e.g., frog, 

 58). In short, the conjugation of two 

 paramecia is the simplest form of fer- 

 tilization known among animals, just 

 as that of Sphaerella, molds, and Spi- 

 rogyra is the simplest in plants. Other 

 kinds of one-celled animals are known 

 to undergo conjugation. 



268. Physiology of Paramecium. We 



are now ready to survey the life-activities of Paramecium. 



A B 



FIG. 90. Division of a 

 Stentor (trumpet-ani- 

 malcule), n, bead-like 

 nucleus; v, vacuole. 

 (From Hatschek.) 



