CHAPTER III 



CELLS AND THEIR PARTS 



CELLS— CILIA AND FLAGELLA— PL ANT- CELLS 

 —COLONIES— INDIVIDUALS 



*' Wherefore one should not be childishly contemptuous of the 

 most insignificant animals. For there is something marvellous 

 in all natural objects." Aristotle. 



Cells 



1 HE Amoeba, as we have seen above, is one of the simplest 

 of those animals which consist of one single cell, but there 

 are an incredible nmnber of different forms of unicellular 

 animals, many of them of the greatest complexity and of 

 an infinite variety of shape. Some of them secrete skeletons 

 which may be either of chalk or of flint. If you pound up, 

 not too finely, a piece of chalk and examine it under a 

 microscope, you 

 will see a number 

 of little chalky 

 shells, some of 

 them of many 

 chambers, which 

 were all secreted 

 by a single cell 

 with man}^ nuclei 

 or one. The im- 

 mense quantity of 

 these little calcareous shells is shown b}?^ the fact that some 

 of our Chalk measures, e.g. those of Norfolk, have an average 

 thickness of some 1450 feet, and the huge areas of the 

 bottom of the ocean are covered with such minute shells, 

 forming a greyish-brown paste, known as Globigerma ooze. 

 Nearly fifty million square miles of the whole surface of the 

 bottom of the deep sea is covered with such chalk}^ shells. 

 Another 2,290,000 square miles of the ocean bed is floored 

 with flinty shells formed by other unicellular organisms. The 



Fio. 3. A shell of Globigerina. After Brady. 



