64 Elementary Biology. 



unicellular organism usually possessing chlorophyll and a 

 cellulose cell-wall ; that is to say, an extremely simple 

 plant. 



It is important to notice that this classification into uni- 

 cellular plants and unicellular animals is a very loose one, 

 and that no doubt many of the organisms at present classed 

 as doubtfully vegetal or animal in their relationships, are in 

 reality transition stages between the two types, or forms 

 which represent the generalised type from which both ani- 

 mal and vegetal have been derived. It is further worthy 

 of note that the presence or absence of chlorophyll is a 

 more important distinction than the presence or absence of 

 a cellulose capsule, since the capsule of cellulose, or of a 

 substance chemically related to cellulose, is developed, at 

 least temporarily, in not a few Protozoa. We have already 

 seen (p. 38) that chlorophyll is far more important from 

 a physiological than from a morphological point of view ; 

 hence we may say that the distinction between unicellular 

 animals and unicellular plants is mainly a physiological one. 



Unicellular organisms make up, however, only a small 

 proportion of the organisms on the earth's surface. By far 

 the majority of organisms are composed of a vast number 

 of cells which are specially modified for the performance 

 of special functions that is to say, differentiated in the 

 mariner already fully explained. Such organisms are said 

 to be nmlticellular, and among these organisms there are 

 several well-marked differences which enable us at a glance 

 to say whether any given organism is a plant or an animal. 

 Such differences are the presence or absence of chlorophyll, 

 the presence or absence of cellulose, the possession or not 

 of the power of movement, the method of feeding, and 

 character of the foodstuffs, &c. We divide these multi- 

 cellular organisms into multicellular plants, or Metaphyta, 

 and multicellular animals, or Metazoa. Even here, how- 

 ever, the lowest group of Metaphyta includes forms which, 

 from their possessing the power of motion, have been often 



