4 HAND-BOOK OF PHYSIOLOGY. 



are freely produced in vegetable structures, although they form a very 

 much smaller proportion of the whole organism than cellulose or starch. 

 And while the presence of the latter in animals is much more rare than 

 is that of the former in vegetables, there are many animals in which 

 traces of it may be discovered, and some, the Ascidians, in which it is 

 found in cpnsiderable quantity. 



(3. ) Inherent power of movement is a quality which we so commonly 

 consider an essential indication of animal nature, that it is difficult at 

 first to conceive it existing in any other. The capability of simple motion 

 is now known, however, to exist in so many vegetable forms, that it can 

 no longer be held as an essential distinction between them and animals, 

 and ceases to be a mark by which the one can be distinguished from the 

 other. Thus the zoospores of many of the Cryptogamia exhibit ciliary 

 or amoeboid movements (p. 8) of a like kind to those seen in animalcules; 

 and even among the higher orders of plants, many, e. g., Dioncea Mus- 

 cipula (Venus's fly -trap), and Mimosa Sensitiva (Sensitive plant), exhibit 

 such motion, either at regular times, or on the application of external 

 irritation, as might lead one, were this fact taken by itself, to regard 

 them as sentient beings. Inherent power of movement, then, although 

 especially characteristic of animal nature, is, when taken by itself, no 

 proof of it. 



(4.) The presence of a digestive canal is a very general mark by 

 which an animal can be distinguished from a vegetable. But the lowest 

 animals are surrounded by material that they can take as food, as a plant 

 is surrounded by an atmosphere that it can use in like manner. And 

 every part of their body being adapted to absorb and digest, they have 

 no need of a special receptacle for nutrient matter, and accordingly have 

 no digestive canal. This distinction then is not a cardinal one. 



It would be tedious as well as unnecessary to enumerate the chief dis- 

 tinctions between the more highly developed animals a*nd vegetables. 

 They are sufficiently apparent. It is necessary to compare, side by side, 

 the lowest members of the two kingdoms, in order to understand rightly 

 how faint are the boundaries between them. 



