SOME MOOT POIXTS IX XATURAL HISTORY. 261 



higher animals, in which food was received and digested. 

 Associated with this nutritive system was another set 

 of organs, namely, that devoted to the circulation of the 

 nutritive fluid or blood, which had been prepared by the 

 action of the digestive apparatus. Now, whilst it is true 

 that animals, as a rule, possess such an internal cavity or 

 stomach, and in the case of higher animals a circulatory 

 system likewise, it is equally clear that no distinction 

 between animals and plants can be founded upon this 

 observation. Many animals, of by no means the lowest 

 grade, for example, want a mouth and digestive system. As 

 a rule, all parasitic animals obtain their food by imbibing 

 the fluids of their hosts, and, like plants, can subsist on fluid 

 or gaseous matter only. The need for distinct and indepen- 

 dent organs of digestion is superseded by the parasite's 

 habits of dependence on its host; and thus parasitism in 

 lower, as in higher life, is seen invariably to produce de- 

 gradation and retrogression in the dependent being. But 

 it is also a curious and notable fact that whilst in the lowest 

 animalcules, represented as already remarked by mere specks 

 of protoplasm (Fig. 41), no digestive or indeed any other 

 organs are to be descried. Certain animals of tolerably high 

 organisation may present exceptions to the rule of organi- 

 sation even in their own species, by exhibiting no traces 

 of a digestive system. Thus, whilst the female rotifers or 

 ;; wheel animalcules " possess a complicated organisation, 

 including a digestive system, sense-organs, and other vital 

 apparatus, the male animalcules possess no digestive organs, 

 and exist probably by the imbibition of fluid matters. 

 Hence, if the presence of an internal cavity or digestive 

 sac is to be regarded, as it was by Cuvier in 1828, as a 

 distinctive character of animals, we have no alternative but 

 to exclude the male rotifers and most parasites from the lists 

 of zoologists ; the latter procedure involving an idea which 

 of course cannot for a moment be entertained, especially 

 when distinctions of apparently greater weight have gone 

 by the board before the levelling assaults and tests of recent 

 research. 



