2 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 Fungi, 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 
othersoluble compound; thus they can live on liquid inorganic food 
