4 INTRODUCTION. 
vous system, and many of the latter possess the power of active locomo- 
tion. In determining, therefore, the nature of these ambiguous organ- 
isms, the following are the chief points to be attended to :— 
firstly, As to mere form or external configuration, no certain rules can 
be laid down for separating animals and plants. Many of the lower 
plants, either in their earlier stages of existence or when grown up, are 
exactly similar in form to some of the lower animals. This is the case, 
for example, in some of the Alege, which closely resemble some of the 
Infusorian animalcules. Many undoubted animals, again, are rooted to 
solid objects in their adult state, and are so plant-like in appearance as 
to be always popularly regarded as vegetables. This is the case with 
many of the so-called hydroid zoophytes, such as the sea-firs, and also 
with the much more highly organised sea-mats (/Zustra), all of which 
are usually regarded as sea-weeds by seaside visitors. ‘This is also, but 
less strikingly, the case with the corals and sea-anemones, of which the 
latter are often spoken of as ‘‘sea-flowers.” 
Secondly, No decided distinction can be drawn between animals and. 
plants as to their minute ézternal structure. Both alike consist essen- 
tially of minute solid particles (molecules or granules), of cells, or of 
fibres. 
Thirdly, As regards chemical composition, there are some decided, 
though not universal, differences between plants and animals: Asa 
general rule, it may be stated that plants exhibit a decided predominance 
of what are known to chemists as ‘‘ ternary compounds ”—that is to say, 
compounds which, like sugar, starch, and cellulose, are composed of the 
three elements, carbon, hydrogen, and oxygen. ‘They are, compara- 
tively speaking, poorly supplied with ‘‘ quaternary” compounds, which 
contain the fourth element, nitrogen, in addition to the three first 
mentioned. Animals, on the other hand, are rich in quaternary 
nitrogenised compounds, such as albumen or fibrine. Still in both 
kingdoms we find nitrogenised and non-nitrogenised compounds, and 
it is only in the proportion which these bear to one another in the 
organism that animals differ in any way from plants. The most 
characteristic of all vegetable compounds is the one known as cellulose, 
very nearly allied in its chemical composition to ordinary starch. As 
a general rule, it may be stated that the presence of an external envelope ~ 
of cellulose in any organism raises a strong presumption as to its vege- 
table nature. Still cellulose is not exclusively confined to plants, as 
was at one time believed. It is now well known that the outer cover- 
ing of the so-called sea-squirts or Ascidian Molluscs contains a large 
quantity of cellulose (as much as 60 per cent in some cases); and 
recent researches seem to prove that this substance is present also in 
some of the lower forms of animal life. Another highly characteristic 
vegetable product is chlorophyll, the green colouring-matter of plants. 
Any organism which exhibits chlorophyll in any quantity as a proper 
element of its tissues is most probably vegetable. In this case also, 
however, the presence of chlorophyll cannot be regarded as a certain 
