362 THE WORLD OF LIFE 



diminish so as to change the whole shape in an hour ov two. 

 But more curious is its power of absorbing any particles of 

 organic matter that come in contact with it by gradually en- 

 closing them in its substance, wdiere after a time they dis- 

 appear. The Amoebae are found in stagnant water full of or- 

 ganic matter, and if they are transferred to pure waiter they 

 soon diminish in size, proving that they require food and can 

 digest it. The '' contractile vacuole " is believed to have the 

 function of expelling the carbonic acid gas and other waste 

 products of assimilation. 



This Amoeba is one of the simplest forms of the lowest 

 branch of the animal kingdom, the one-celled animals or 

 Protozoa ; all other animals being classed as Metazoa, as they 

 are entirely built up of separate cells, which in all the more 

 complex forms are countless millions in number. Every part 

 of our bodies, from blood to muscles and nerves, from bones 

 to skin, hair, and nails, is alike constructed of variously modi- 

 fied cells. 



It might be thought that animals consisting of single cells 

 could not be very numerous or very differently organised. 

 Yet they are grouped into five classes, the first, Rhizopoda, 

 comprising not only many kinds of Amoeba?, but the beauti- 

 ful Foraminifera, whose exquisite shells are such favourite 

 microscopic objects. They are single amoeboid cells which yet 

 have the power either of building up shells of small inorganic 

 particles, or of secreting the more beautiful shells which seem 

 to mimic the forms of those of the higher Mollusca. The 

 fossils called Xummulites were Foraminifera with flat coiled 

 shells, forming great masses of Eocene limestone. They are 

 the largest of all, some equalling a half-crown in size. 

 Radiolaria are rhizopods having a beautiful siliceous skele- 

 ton, and often living in colonies. Another class, the Mastigo- 

 phora, have extremely varied shapes, often like sea-weed or 

 flowers, having long, slender, whip-like processes. These and 

 hundreds of other strange forms are still essentially single 

 cells, though often grouped together for a time, and they all 



