336 THE WORLD OF LIFE CHAP. 



careful watching, these are seen to increase or diminish so as 

 to change the whole shape in an hour or two. But more 

 curious is its power of absorbing any particles of organic 

 matter that come in contact with it by gradually enclosing 

 them in its substance, where after a time they disappear. 

 The Amcebae are found in stagnant water full of organic 

 matter, and if they are transferred to pure water 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 Amceba 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 modified 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 Amcebae, but 

 the beautiful 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 Nummulites 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 skeleton, and often living in 

 colonies. Another class, the Mastigophora, 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 increase either by 

 division or by giving off buds, which rapidly grow into the 

 perfect form. 



