268 ZOOLOGICAL PHILOSOPHY 



It follows from the foregoing that no living body, which lacks an 

 alimentary cavity, ever has any digestion to perform ; and since all 

 digestion works on compound substances and breaks down the aggre- 

 gation of the food molecules into solid masses, it results that such 

 living bodies as have no digestion, can only feed on fluid, Uquid or 

 gaseous material. 



This applies to all plants ; they have no digestive organ, nor as a 

 matter of fact do they have any digestion to perform. 



Most animals, on the contrary, have a special organ for digestion ; 

 but this faculty is not, as has been alleged, common to all animals, 

 and cannot be cited as one of the characters of animahty. The 

 infusorians indeed do not possess it ; and we should vainly seek an 

 ahmentary cavity in a monas, volvox, proteus, etc. ; there is none to 

 be found. 



The faculty of digesting is then only common to the greater number 

 of animals. 



Respiration. This is the second of the faculties peculiar to certain 

 animals, for it is less general than digestion ; its function is carried 

 on in a distinct special organ, which varies greatly in different races 

 and different requirements. 



This function consists in a restoration of the essential fluid, which 

 in these individuals becomes too rapidly degraded ; a restoration 

 for which the slower alternative of food is not sufficient. The restora- 

 tion in question is effected in the respiratory organ by means of the 

 contact of a special fluid that is breathed in, and decomposes and 

 communicates restorative principles to the individual's essential fluid. 



In those animals whose essential fluid is quite simple and only moves 

 slowly, the degradation of this fluid is also slow, and then the method 

 of food alone suffices for the restorations ; the fluids capable of provid- 

 ing certain necessary restorative principles penetrate into the individual 

 by this route and also by absorption ; and their influence is sufficient 

 without any need for a special organ. Hence the faculty of breathing 

 by a special organ is not necessary to these living bodies. This is the 

 case with all plants and also with a considerable number of animals, 

 such as those that compose the class of infusorians and that of polyps. 



The faculty of breathing then should only be attributed to those 

 living bodies that possess a special organ for the purpose ; for if 

 those which have no such organ require for their essential fluid any 

 influence analogous to respiration (which is very doubtful), they 

 apparently derive it through some slow general route like that of 

 food or of absorption through external pores, and not by a special 

 organ. Hence these living bodies do not breathe. 



The most important of the restorative principles furnished by the 



