32 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



nucleus, the latter often being difficult to see unless the 

 specimens are stained. The slender stalk is made up of a 

 clear outer portion and a denser contractile inner rod. 



The Vorticellae multiply by longitudinal division, or fission. 

 In this process a cleft first appears at the distal end of the bell- 

 shaped body and gradually deepens until the original body is 

 divided quite in two. The stalk also divides for a very short 

 distance. One of the new bell-shaped bodies develops a 

 circlet of cilia near the stalked end. After a while it breaks 

 away and swims about by means of this basal circlet of cilia. 

 Later it settles down, becomes attached by its basal end, loses 

 its basal cilia and develops a stalk. Conjugation sometimes 

 occurs between two individuals. Under certain conditions 

 there is produced, by repeated divisions, small free-swimming 

 forms, one of which may meet one of the large stalked forms 

 and be completely absorbed by it. This differs from the proc- 

 ess of fertilization in the Paramoscia in which the union was 

 only temporary, and presents an even more striking analogy 

 with the process of sexual reproduction occurring in the higher 

 animals. 



Marine Protozoa. The Protozoa are more abundant in 

 the ocean than they are in fresh water. Although the ocean 

 water may appear to the unaided eyes as clear and free from 

 living things, yet a microscopical examination will show it to 

 be swarming with minute animals and plants. These are 

 found at all depths, from the surface to the deepest parts of 

 the ocean, and are interesting not only because they repre- 

 sent the lowest, simplest and doubtless earliest kinds of ani- 

 mals that appeared on the earth, but because they furnish, 

 together with the Protophyta, or one-celled plants, directly or 

 indirectly, food for all of the other animals of the sea. As we 

 study some of the representatives of the higher groups of ocean 

 animals we shall see that many of them are particularly adapted 

 by structure and habit for feeding on these minute organisms, 

 and that they in turn serve as food for other animals, so 

 that finally all of the animal life in the ocean becomes de- 

 pendent on the one-celled organisms for their food. This is 

 one of the reasons for believing that the Protozoa were the first 



