I 2 MANUAL OF ZOOLOGY. 



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. 



Finally, it is worthy of notice that it is in their lower and 

 not in their higher developments that the two kingdoms of 

 organic nature approach one another. No difficulty is ex- 

 perienced in separating the higher animals from the higher 

 plants, and for these universal laws can be laid down to which 

 there is no exception. It might, not unnaturally, have been 

 thought that the lowest classes of animals would exhibit most 

 affinity to the highest plants, and that thus a gradual passage 

 between the two kingdoms would be established. This is not 

 the case, however. The lower animals are not allied to the 

 higher plants, but to the lower ; and it is in the very lowest 

 members of the vegetable kingdom, or in the embryonic and 

 immature forms of plants little higher in the scale, that we 

 find such a decided animal gift as the power of independent 

 locomotion. It is also in the less highly organised and less 

 specialised forms of plants that we find the only departures 

 from the great laws of vegetable life, the deviation being in the 

 direction of the laws of animal life. 



5. MORPHOLOGY AND PHYSIOLOGY. 



The next point which demands notice relates to the nature 

 of the differences between one animal and another, and the 

 question is one of the highest importance. Every animal 



