206 SCIENCE FROM AN EASY CHAIR 



sculptured coat of glass-like silica (quartz), which resists 

 destruction and persists long after the protoplasm is 

 dead and washed away. They are favourite objects for 

 examination with the microscope on account of their 

 great beauty and variety. 



Those simplest living things which have not got leaf- 

 green to enable them to feed on mineral food must 

 unless they are parasites (as many important kinds are) 

 get their food, as do bigger animals, by feeding on the 

 solid substance of other living things. All living things 

 are, in fact, ultimately dependent on the green plants 

 whether microscopic or of larger kinds not only for 

 food, but for oxygen gas. If you could take away green 

 plants altogether from the world, the animals would eat 

 one another and use up the oxygen gas of the atmo- 

 sphere, and at the last there would be a few only of the 

 strongest left, like the last survivor of the shipwrecked 

 crew of the Nancy Bell, and even they would be suffocat- 

 ing for want of oxygen. The single cells, which are 

 independent animalcules, and feed like animals on whole 

 creatures smaller than themselves, or on bits of the fresh 

 substance of other animals or of plants, are of extra- 

 ordinary diversity of form and activity. Unlike the 

 unicellular plants, whose food is dissolved in the water 

 in which they live, the single-cell animals of necessity 

 take their food in " lumps " into their inside and digest 

 it, and so their cell-protoplasm has either a soft surface 

 which can take up a food-morsel at any point or it has 

 a firm surface with a definite mouth, or aperture, in it 

 (see Fig. 41) where the mouth is marked by an arrow. 

 Many of them, especially those with soft glutinous pro- 

 toplasm, which extends from the main-mass in long 

 threads or branching processes searching for food-morsels, 

 form marvellous, perforated shells by chemical deposit, 

 either of silica or limestone (Radiolaria and Foraminifers). 



