10 ZOOLOGICAL EXERCISES. 
arranged under four heads: (1) alimentation ; (2) reproduc- 
tion; (3) locomotion; and (4) sensation. In its earliest 
form, as seen in the Protozoa, each cell possesses all these 
functions in an imperfect degree ; but in the Metazoa, where 
the original cell divides into an aggregate, division of labour 
commences. Each cell always retains the function of 
alimentation, but most of them lose the power either of 
reproduction, of movement, or of sensation, and these dif- 
ferentiated cells combine together to form organs, or systems 
of organs, which perform the necessary functions for the 
whole individual. 
Nutriment must be in a fluid state to penetrate to all 
the cells in a tissue, consequently those organisms that con- 
sume solid substances must have some apparatus for con- 
verting it into fluid—that is, for digesting it. This is 
called the alimentary system. In the higher animals, 
digestion consists of three different processes. The food is 
first mixed with saliva, which moistens it and converts some 
of the starch contained in it into sugar; it is then acted on 
in the stomach by the gastric juice, a mixture of pepsin and 
acid, which dissolves the nitrogenous substances, and reduces 
the food to chyme; lastly, in the small intestine, the chyme 
receives the secretions of the pancreas and liver, which are 
alkaline, and which complete the conversion of starch into 
sugar, and dissolve the fatty substances* The chyme is 
now changed into chyle, and is in a fit state to be 
absorbed into the circulatory system, which distributes 
the nutrient fluid to the tissues, and carries off the 
products of decomposition. 
A plant manufactures protoplasm by the combination 
of ammonia, water, and carbonic acid, thus :— 
* It is by no means certain that the so-called liver in the Invertebrata 
has the samme frnctions that it has in the Vertebrata. 
