AMCEBA, PARAMCECIUM AND VORTICELLA 47 



many are included in the body of any one of the complex 

 animals. 



Protoplasm. — The protoplasm, which is the essential liv- 

 ing substance of the typical animal cell and hence of the 

 whole animal body, is a substance of very complex chemical 

 and physical make-up. The most important thing about 

 the chemical constitution of protoplasm is that there are 

 always present in, it certain complex albuminous substances 

 called proteids which are never found in inorganic bodies, 

 although the elements that compose these substances as well 

 as all the rest of the protoplasm are the familiar ones, car- 

 bon, nitrogen, hydrogen, oxygen, sulphur, phosphorus, 

 potassium, sodium, etc. The atoms in a single proteid 

 molecule often number more than a thousand, and the mole- 

 cules are very large. But chemists have yet to find out a 

 great deal about these complex albuminous compounds. 



In addition to the proteids protoplasm usually contains 

 certain native albumins and certain other characteristic 

 compounds known as carbohydrates and fats (which differ 

 essentially from the albuminous substances in lacking nitro- 

 gen as a composing element). There are also various salts 

 and gases and always water to be found in living substances. 

 Water is absolutely necessary to the physical condition of 

 half fluidity which gives to protoplasm its essential capacity 

 for motion on itself. The commoner salts found in living 

 substances are compounds of chlorine as well as the car- 

 bonates, sulphates, and phosphates of the alkalies and al- 

 kali earths, especially common salt (sodium chloride), 

 potassium chloride, ammonium chloride, and the carbon- 

 ates, sulphides, and sulphates of sodium, potassium, magne- 

 sium, ammonium, and calcium. The gases found in living 

 matter are oxygen and carbon dioxide. These, when not 

 in chemical combination, are almost always dissolved in 

 water, although rarely they may be in the form of gas bubbles. 



The physical constitution of protoplasm seems to be that 



