THE PHENOMENA OF LIFE. 11 



between vegetables and animals, even in their lowest forms; for associ- 

 ated with the protoplasm of the former is a considerable amount of 

 cellulose, a substance closely allied to starch and containing carbon, 

 hydrogen, and oxygen only. The presence of cellulose in animals is 

 much more rare than in vegetables, but there are many animals in which 

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

 found in considerable quantity. The presence of starch in vegetable 

 cells is very characteristic, though not distinctive, and a substance, gly- 

 cogen, nearly allied in composition to cellulose, is very common in the 

 organs and tissues of animals. 



(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 mo- 

 tion 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 ani- 

 mals, 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. 4) of a like kind to those seen in 

 amoebae; and even among the higher order of plants, many, e.g., Dioncea 

 Muscipula (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 

 distinctions between the more highly developed animals and vegetables. 

 They are sufficiently apparent. 



In passing, it may be well to point out the main distinctions between 

 animal and vegetable cells. 



It has been already mentioned that in animal cells an envelope or 

 cell-wall is by no means always present. In adult vegetable cells, on the 

 other hand, a well defined cellulose wall is highly characteristic; this, it 

 should be remembered, is non-nitrogenous, and thus differs chemically 

 as well as structurally from the contained mass. 



Moreover, in vegetable cells (Fig. 8, B), the protoplastic contents of 

 the cell fall into two subdivisions: (1) a continuous film which lines the 



