SIMPLEST ANIMAL8 319 



colonies forming inside the old colony. Each of the new 

 colonies is started by one cell (individual) moving inward 

 and dividing up into a large number of cells, which form a 

 new colony. Finally, the outer sphere breaks and releases 

 the new colonies. 



Sometimes Volvox has sexual reproduction. Certain cells 

 enlarge and become egg-cells. Some other cells form sperm- 

 cells. Each fertilized egg-cell divides into a large number 

 of cells which form a new colony. 



Biologists are disagreed as to whether Volvox is an animal 

 or a plant. It is commonly described in textbooks of both 

 botany and zoology. The truth is that it is one of many 

 small organisms some living singly and some in colonies 

 which so combine characters of both animals and plants that 

 they may be said to be near or on the boundary line between 

 the animal and plant kingdoms. Volvox behaves like an 

 animal, but its nutrition is that of a green plant. In fact, 

 each cell of a Volvox colony is comparable to a single Sphae- 

 rella ( 238) . It is probably a colony of one-celled plants, one 

 of the Algae. 



However, whether Volvox is animal or plant is a point of 

 little importance, for it is an organism illustrating colonial life 

 of one-celled living things so well that it deserves to be studied 

 in connection with both botany and zoology. 



Volvox, like the colonial bell-animalcules, has some cells 

 for nutrition, and some specialized for reproduction. In the 

 next animals to be studied we shall find this division of labor 

 more fully developed. 



Classes of Protozoa (One-Celled Animals) 



Sarcodina simple masses of protoplasm, with pseudopodia. 

 Amreba. 



Sporozoa spore-animals ; producing spores. Malaria germs. 



Mastigophora ; with flagella for swimming. Volvox. 



Infusoria with cilia for swimming and collecting food. Para- 

 mecium, Vorticella. 



