THE DIFFERENCES BETWEEN ANIMALS AND PLANTS. 11 



of the animal are altered in form, whilst those of the plant 

 retain more or less their original appearance. The cell-wall, 

 too, of the animal is nitrogenous, that of the plant is non- 

 nitrogenous. But all this only applies to the higher plants and 

 animals : it cannot apply to those unicellular forms, where, as we 

 see in Amoeba, there is no cell-wall at all. It is often thought 

 that we can tell a plant hy its green colouring matter, 

 chlorophyll, but not all plants have this chromatic substance 

 in their tissues ; whilst, on the other hand, some animals — such 

 as Hydra, Bonellia (one of the Worms), and some sea-anemones 

 (Aetinuzoa) — owe their green colour to the presence of this sub- 

 stance. Cellulose is .the substance that forms the cell-wall of 

 plants, and is characteristic of the vegetable kingdom ; but we 

 also find it in the " tests " or cases of those curious marine 

 animals, the sea-squirts or Aseidians. In the higher animals a 

 substance known as cholesterin is found : this was at one time 

 considered a purely animal component, but we now know that 

 it is also found in at least one family of plants, the Leguminosai 

 or Pea and Bean family. Generally speaking, animals are nitro- 

 genous, plants carbonaceous ; but, as in the prior instances, this 

 also will not invariably apply. There are no definite distinc- 

 tions, then, between the animals and plants in regard to their 

 chemical constituents. Perhaps the greatest differences are to 

 be found in the metabolism of organisms. We cannot feed an 

 animal on purely inorganic food, whilst, on the other hand, we 

 can so feed a plant. Both must have salts and water ; but 

 whilst plants can be nourished with the addition of carbon 

 dioxide and nitrates of ammonia, an animal must have nitrogen- 

 ous and carbonaceous matter in some organic form and not in a 

 mineral form. An animal absorbs oxygen and gives out CO2 ; 

 a plant exhales oxygen which is derived from the absorbed COj. 

 Thus we see that there are differences between the plant and 

 the animal, but that many of them do not invariably hold good. 

 There are forms of life which we may fairly say bridge over 

 the great hiatus that separates the horse from tlae grass upon 

 which it feeds. 



