THE DIFFERENCES BETWEEN ANIMALS AND PLANTS. 11 



botanists as plants, wMlst the zoologist includes them in the 

 Protozoa. It may he said, speaking generally, that animals 

 are capable of free movement and that plants are fixed ; but 

 when we examine some of the simplest forms of life this 

 distinction will be found untenable. Animals are endowed with 

 sensation, plants are not, as a rule ; but such plants as Drosera, 

 Venus' s fly-trap, &c., surely have this phenomenon developed. 

 Animals have their organs internal, their absorbent surface 

 inside ; plants have external organs, and the absorbent surface 

 also external. Think for a moment of the Tapeworms (Cestoda), 

 which obtain their nourishment by osmosis through the skin, 

 and we shall at once see that this again will not hold good. 



"When we compare the tissues of an animal with the tissues 

 of a plant, then we observe greater differences. The cells 

 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 by 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 

 {Actinozoa) — 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 Ascidians. 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 LeguminoscB 

 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- 



