Animals and Plants. i8i 



stationary, the symmetry, if marked, is rather radial, such as 

 sea anemones, etc. The process of feeding, too, is different : 

 solid food is taken in, which is rendered diffusible by the 

 processes of digestion ; it is thus the internal structures which 

 tend to become complicated. There is no need of so much 

 surface. The animal either goes in search of its food by 

 moving from place to place, or — if fixed — it possesses mechan- 

 isms, such as the tentacles of hydra, for capturing it. It is 

 interesting to note that certain parasitic animals (some 

 Crustacea), which are" imbedded among plenteous nutriment in 

 the bodies of their hosts, often tend to grow in length and 

 irregularly, like plants. 



2. Related to the last-mentioned difference between animals 

 and plants is the fact that all the Metazoa — the animals above 

 the unicellular creatures, which will be dealt with later — are 

 either permanently or temporarily two-layered sac-like creatures ; 

 the Metaphyta are never so. 



3. It is commonly, but in some ways with insufficient 

 accuracy, stated that animals and plants differ in the prevailing 

 cellulose cell-wall of the latter and its absence in the former. 

 The statement is, indeed, perfectly true, but the emphasis is 

 wrongly laid. This difference is, in the first place, that in 

 animals the motile phase is the more prevalent, in plants the 

 encysted. It will be remembered that the amoeba is for the 

 most of its life a naked mass of protoplasm; but that on 

 occasions it becomes encysted. On the other hand, the 

 prevailing conditions of organisms on the same plane as 

 amceba, such as the unicellular Alga, Hcematococms, is usually 

 enveloped in a cell-wall; on occasions this is thrown off, and 

 the creature moves through the water a naked mass of proto- 

 plasm. Animals are, however, to be distinguished from plants 

 by the fact that their cell-wall, when present as a distinct 

 structure, is not made of cellulose.^ In plants it is, as a rule. 



It will be gathered from the foregoing brief account of the 

 main differences between animals and plants that there is no 



' It must be remembered that cellulose is not absent from animals ; it 

 occurs, for example, as a constituent of the test of Ascidians. Bui, when 

 present, it does not form the walls of individual cells. 



