n BACTERIA 257 



mode of nutrition, and resembles that which occurs in certain 

 plants which contain no chlorophyll — the fungi. It will be 

 seen that the main difference between saprophytic and 

 holozoic nutrition is that in the former digestion, i.e., the 

 process of rendering food-stuffs soluble and diffusible, takes 

 place outside the body, so that constructive metabolism can 

 begin at once. 



In the Monad, the characters are again conflicting : the 

 probable absence of cellulose, the presence of a contractile 

 vacuole, and the flagella all have an "animal" look, but 

 the mode of nutrition is that of a fungus. 



Decomposition, as already stated (p. 152), is due in the first 

 instance to the action of certain other minute organisms, 

 known as Bacteria, which appear in the earlier stages of the 

 putrefaction of an organic infusion. The nutrition of these 

 organisms, like that of Monads, is usually saprophytic and 

 the movements are generally ciliary ; but as they have a 

 definite cell-wall and no contractile vacuole, they are almost 

 universally classed amongst plants, while Monads are as 

 constantly included in the animal kingdom. 



We see then that while it is quite easy to divide the 

 higher organisms into the two distinct groups of plants and 

 animals, any such separation is by no means easy in the 

 case of the lowest forms of life. It was in recognition of 

 this fact that Haeckel proposed, many years ago, to institute 

 a third " kingdom," called Protista, to include all unicellular 

 organisms. Although open to many objections in practice, 

 there is a great deal to be said for the proposal. From the 

 strictly scientific point of view it is quite as justifiable to 

 make three subdivisions of living things as two : the line 

 between animals and plants is quite as arbitrary as that 

 between protists and plants or between protists and animals, 

 and no more so : the chief objection to the change is that it 



Pract. Zool. S 



