PROTOZOANS. 267 



521. In most cases they have neither mouth nor 

 stomach, and they are exceedhigly minute and mostly 

 microscopic. They are doubtless more numerous 

 than all the other animals of the globe, for they 

 live in immense numbers in every ditch and pool, 

 every stream, pond, and lake, and almost every part 

 of the sea. There is scarcely a drop of stagnant water 

 that is not inhabited by some of them. They were 

 exceedingly abundant in the past ages of the world ; 

 for their skeletons, or hard parts, fill the rocks in many 

 places, and rocky strata hundreds of feet in thickness 

 are wholly made up of their remains. 



One group of the Protozoans is called Infusoria, 

 from having first been found in vegetable infusions, 

 that is, in liquids in which plants have been soaked. 

 These are very abundant in fresh water ponds, etc. 

 Of these, Vorticella, Figure 513, is a well-known kind. 



There is another group called Rhizopods, — a word 

 meaning roof-/£ct, — because they throw out fibers or 

 root-like appendages, as in Figures 519, 520. Many of 

 these have a shell, and are often called Foraminifers 

 from the pores or foramens in the shell, through which 

 the appendages just mentioned are thrust out. The 

 vast chalk-beds of Europe are almost wholly made of 

 the shells of Rhizopods, which are so minute that a 

 million are contained in a cubic inch of the chalk, 

 so that, small as these creatures are, they have played 

 a part in the building of the world. They live mostly 

 in the ocean near the surface and their shells, as they 

 die, are constantly falling to the bottom. The floor 

 of the ocean, away from the immediate neighborhood 

 of the land is covered with a soft mud or ooz-c mainly 

 composed of these shells. The Nummulite, Figure 



