XXI 

 THE SIMPLEST LIVING THINGS 



IN old times, if one wanted to compare a man to the 

 humblest and simplest of animals, one called him 

 "a worm." But really a worm is a very elaborate 

 creature, with skin, muscles, blood-vessels, kidneys, 

 nervous system, pharynx, stomach, and an intestine, 

 and is built up by hundreds of thousands of protoplasmic 

 cells. Shakespeare got nearer the mark when he made 

 one- of his uncompromising professional "murderers" 

 exclaim, as he stabbed the young Macduff to the heart, 

 " What, you egg ! " An egg is a single cell or corpuscle of 

 protoplasm, and the simplest living things are of the 

 same structure mere units, single corpuscles of proto- 

 plasm, often less than the one-thousandth of an inch in 

 diameter, and invisible except with the microscope, 

 though in some cases big enough to be seen by the 

 naked eye as they swim or crawl in a glass of pond- 

 water. Many thousands of kinds of these simplest 

 animals and plants have been carefully recorded, distin- 

 guished from one another, and named by naturalists. 



Many of these unicellular animals (or " Protozoa ") 

 crawl by a curious irregular flowing movement of the 

 viscid tenacious protoplasm of which they consist. 

 There is no firm coat or cell-wall, only the thinnest 

 pellicle on the surface. The Proteus-animalcule (Fig. 



