78 THE VITAL FUNCTIONS 



different in different kinds of organism, though it must 

 always contain the necessary chemical elements, but 

 the food of protoplasm is always of the same nature. 



Four main types of feeding are often distinguished : 

 holozoic, in which the organism ingests solid organic 

 food (characteristic of animals) ; holophytic, in which 

 the food of the organism consists wholly of inorganic 

 substances, including carbon dioxide (characteristic of 

 green plants) ; saprophytic, in which organic food is 

 obtained from the dead body or from the products of 

 some other organism in a liquid form ; and parasitic, 

 in which organic food is obtained direct from the living 

 body of some other organism, but this " host " organism 

 is not immediately killed. The mode of nutrition of 

 saprophytic and parasitic organisms is not essentially 

 different. It is of no essential importance whether 

 the liquid organic substances, provided they are suitable 

 as food, come from a living or from a dead body or 

 product ; and many species of fungi, for instance, may 

 be both saprophytic and parasitic (see Chapter XI), 

 though in many cases species are closely adapted to 

 one or the other mode of life, Animal parasites may 

 have mouths, and suck or devour organic liquids or 

 tissues of the living body of the host, or they may, 

 like the tapeworm, have no mouths, and absorb liquid 

 organic food through their body wall. 



(2) Assimilation.' — This word hterally means ' ' making 

 like," and in its strict sense is applied to the process 

 of incorporation of the final foodstuffs in^ the specific 

 protoplasm of the organism. It is, in fact, the actual 

 feeding of the protoplasm. We know practically 

 nothing of how this is done The incorporation of the 

 carbon derived from the carbon dioxide of the air in 



' Latin similis, like. 



