194 ON THE DISTINCTION OF AN ANIMAL 



ble that the excessive minuteness of the infusoria may ac- 

 count for our not having yet observed an intestinal cavity 

 in them : and with respect to the polypes, the pretended ob- 

 jection is in reahty none; as though the intestine of these 

 animals maybe turned inside out, yet the food is constantly 

 received into it, and the nourishment consequently absorb- 

 ed by that surface which may happen to be internal. 



The second character seems much more exception- 

 able; for M. Biot is said to have discovered that several 

 species of Coleopterous insects may exist in a vacuum 

 without inconvenience, and it ^vould above all be diflftcult 

 to apply the rule of the absorption of oxygen to the intes- 

 tinal worms. 



To the third character it may be objected that a nerv* 

 oas system has not yet been detected in all animals ; and 

 also, that several vegetables, such as the sensitive plant, 

 possess irritable properties; finally, that all plants appear 

 to direct their organs to what is natural and beneficial to 

 them in such a manner as to render it at least very ques- 

 tionable whether they be not endowed with sensation. To 

 these objections it may be answered, in the first place, that 

 in all animals where a nervous system has been detected, 

 their sensation has been discovered to depend on it ; we 

 are hence led to assume that all animals in Avhich sensa- 

 tion is observable must have it depending on a nervous 

 &ystem. 



It may be proper to say a few words here on what we 

 mean by a nervous system, which is thus made the great 

 characteristic of an animal. Filaments of a peculiar me- 

 dullary substance dispersed throughout an organized body 

 on different plans are called the Nerves. The plan of the 

 dispersion of these filaments is called the N&vous System, 



