ZOOLOGY 



composing the body sometimes remain in connection with one 

 another, but they never form definite tissues, and the cells of 

 such an aggregate are physiologically distinct and independent 

 of one another, the whole forming a colony of unicellular 

 beings. 



The organic world has developed in two diverging direc- 

 tions, one corresponding to the animal the other to the 

 vegetable kingdom, and though there is no difficulty in 

 distinguishing the higher forms of these two kingdoms, it is 

 . often by no means an easy matter to determine whether some 

 of the lower forms should be grouped with the plants or with 

 the animals ; hence any scheme of classification is largely 

 dependent on individual opinion. There are a number of 

 characters which if met with in an organism would justify us 

 in claiming it as an animal, but in many cases one or more of 

 these animal features are absent, and again other features may 

 be present which, as a rule, are only found in plants, so that it 

 becomes at once evident that the line between animals and 

 plants, at any rate in their lowest forms, represents no scientific 

 frontier, but is an arbitrary boundary which is apt to be shifted, 

 now forward now backward, according to the opinion of the 

 various investigators. 



The most important morphological difference between plants 

 and animals is perhaps the presence of a cellulose coat which 

 encloses, at any rate during some part of its life, the vegetable 

 cell. Cellulose is a substance which has a definite chemical 

 composition, and which, though practically universal in plants, 

 is very rarely met with in animals. Another constituent 

 found in all green plants, but rare in animals, is chlorophyll ; 

 the presence of this enables the plant in sunlight to take 

 in carbon dioxide, which serves as part of its food ; chlorophyll 

 is, however, not found in all plants, the Tungi, an important 

 section of the vegetable kingdom, being devoid of it. 



The physiological differences between plants and animals 

 are more striking than the morphological. Plants can live 

 upon much simpler compounds than animals ; they can absorb 

 their nitrogen in the form of nitrates or simple compounds 

 of ammonia, and their carbon in the form of carbonic acid, or some 

 other soluble compound; thus they can live on liquid inorganic food 



