10 MANUAL OF ZOOLOGY. 



upon plants. All animals, as far as is certainly known, require 

 ready-made proteinaceous matter for the maintenance of exist- 

 ence, and this they can only obtain in the first instance from 

 plants. Animals, in fact, differ from plants in requiring as food 

 complex organic bodies which they ultimately reduce to very 

 much simpler inorganic bodies. Plants, therefore, are the great 

 manufacturers in nature, animals are the great consumers. 



Just, however, as this law does not invariably hold good for 

 plants, certan fungi being in this respect animals, so it is 

 not impossible that a limited exception to the universality of 

 the law will be found in the case of animals also. Thus, in 

 some recent investigations into the fauna of the sea at great 

 depths, a singular organism, of an extremely low type, but 

 occupying large areas of the sea-bottom, has been discovered, 

 to which Professor Huxley has given the name of Bathybius. 

 As vegetable life is extremely scanty, or is altogether wanting, 

 in these abysses of the ocean, it has been conjectured that 

 this organism is possibly endowed with the power otherwise 

 exclusively found in plants of elaborating organic compounds 

 out of inorganic materials, and in this way supplying food for 

 the higher animals which surround it. The water of the 

 ocean, however, at these enormous depths, is richly charged 

 with organic matter in solution, and this conjecture is thereby 

 rendered doubtful. 



Be this as it may, there remain to be noticed two distinc- 

 tions, broadly though not universally applicable, which are 

 due to the nature of the food required respectively by animals 

 and plants. In the first place, the food of all plants consists 

 partly of gaseous matter and partly of matter held in solution. 

 They require, therefore, no special aperture for its admission, 

 and no internal cavity for its reception. The food of almost 

 all animals consists of solid particles, and they are, therefore, 

 usually provided with a mouth and a distinct digestive cavity. 

 Some animals, however, such as the tape-worm and the Gre- 

 garinae, live entirely by the imbibition of organic fluids through 

 the general surface of the body, and many have neither a dis- 

 tinct mouth nor stomach. 



Secondly, plants decompose carbonic acid, retaining the 

 carbon and setting free the oxygen, certain fungi forming an 

 exception to this law. The reaction of plants upon the atmo- 

 sphere is, - therefore, characterised by the production of free 

 oxygen. Animals, on the other hand, absorb oxygen and emit 

 carbonic acid, so that their reaction upon the atmosphere is 

 the reverse of that of plants, and is characterised by the pro- 

 duction of carbonic acid. 



