AMCEBA, VORTICELLA, PARAMECIUM 33 



out again, the edges turn outward and the body expands 

 until it gradually assumes its former shape. 



Vorticella3 reproduce by fission, one of the resultant forms 

 swimming away and developing a stalk for attachment. 

 At times they conjugate, while some individuals may 

 become encysted and divide up into spores or germs 

 which finally escape from the cyst and develop into new 

 individuals. 



General characteristics of the Protozoa. — With the 

 amoeba, we started among the most primitive animals. It 

 may seem strange that the oldest animals are the simplest, 

 but it is true. It is probably due to the fact that these 

 animals have always lived in the water, and the water is 

 probably not very different to-day from what it was ages 

 ago. Hence there has been nothing to bring about change 

 in these animals, and they have remained much the same. 



Again, bear in mind that we have been dealing entirely 

 with one-celled animals. Yet they can do, in a simple, 

 primitive way, much that we can do. They can all move; 

 some slowly, by crawling, as the amoeba; some faster, 

 by thousands of cilia, as the Paramecium; while others, like 

 the vorticella, are attached, yet possess distinct and remark- 

 able movement, within a certain hmit. All collect food and 

 assimilate it and grow as any other animal. Each one 

 breathes, of course in a primitive manner; but, neverthe- 

 less, oxygen is taken in, and carbonic acid given off as 

 regularly as among the higher animals. 



If we jar the slide on which the vorticella is placed, the 

 tiny animal will suddenly roll into a ball and dart back to its 

 place of support. We have already seen how the amoeba re- 

 acts toward food and avoids a grain of sand. Paramecia 

 are sensitive to an electrical current. Hence we see that 



herrick's zool. 3 



