252 PROTOZOANS. 



meaning root-feet — because they throw out fibre or 

 root-like appendages, as in Figures 516, 517. Most 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. 

 The Nummulite, Figure 518, is one of the Rhizopods 

 or Foraminifers, which has a shell half an inch or. 

 more in diameter, in some cases, and divided into 

 chambers which resemble those of a Nautilus or Am- 

 monite. Extensive beds of limestone are made of 

 Nummulites ; that of which the Pyramids of Egypt 

 are built is filled with shells of this sort. The Amoeba, 

 Figure 516, is a Rhizopod which has no shell. It is a 

 simple, almost fluid mass, seen only by the aid of a 

 microscope, and it changes its form almost every mo- 

 ment. It has neither mouth nor stomach, yet on 

 coming to a particle of food it readily closes around 

 it, and digests it, any part of the body being formed 

 into mouth, stomach, or tentacles, as the occasion re- 

 quires ! 



Sponges are protozoans which have been regarded 

 by many as plants, but are now generally considered 

 to be compound animals. They are common in ponds 

 and lakes, as well as in nearly all parts of the sea, and 

 their forms are exceedingly various and often ex- 

 tremely beautiful. Some cover the rocks like a carpet 

 of mosses ; others grow in massive clusters ; others 

 branch like trees and shrubs ; and others still take the 

 form of the most elegant cups, goblets, and vases. 



