n ANIMALS AND PLANTS 167 



broken down, so far, at least, as his mode of stating 

 it goes. And, with the advance of microscopic 

 anatomy, the universaHty of the fact itself among 

 animals has ceased to be predicable. Many 

 animals of even complex structure, which live 

 parasitically within others, are wholly devoid of an 

 alimentary cavity. Their food is provided for 

 them, not only ready cooked, but ready digested, 

 and the alimentary canal, become superfluous, 

 has disappeared. Again, the males of most 

 Rotifers have no digestive apparatus ; as a German 

 naturalist has remarked, they devote themselves 

 entirely to the " Minnedienst," and are to be 

 reckoned among the few realisations of the 

 Byronic ideal of a lover. Finally, amidst the 

 lowest forms of animal life, the speck of gelatinous 

 protoplasm, which constitutes the whole body, has 

 no permanent digestive cavity or mouth, but takes 

 in its food anywhere ; and digests, so to speak, all 

 over its body. 



But although Cuvier's leading diagnosis of the 

 animal from the plant will not stand a strict test, 

 it remains one of the most constant of the dis- 

 tinctive characters of animals. And, if we sub- 

 stitute for the possession of an alimentary cavity, 

 the power of taking solid nutriment into the body 

 and there digesting it, the definition so changed 

 will cover all animals, except certain parasites, 

 and the few and exceptional cases of non-parasitic 

 animals which do not feed at all. On the other 



