NERVOUS SYSTEMS OF INVERTEBRATES 5 
pods will be drawn in which are not at the front end for the 
time being (fig. 1006 D). 
Since a hungry Amceba creeps actively about for a long 
time we are probably justified in concluding that some of its 
movements are spontaneous, and these are probably initiated 
by chemical changes which take place within its body, and may 
be called internal stimuli. 
NERVOUS SYSTEMS OF INVERTEBRATES 
The Ameeba, like most other animalcules, is a single cell or 
structural unit, which has to discharge all the functions of life, 
and does not exhibit the principle of division of labour to the 
same extent as animals belonging to the higher groups, which 
are collectively termed Metazoa, as contrasted with the Ani- 
malcules or Protozoa. Every member of the former group is 
made up of more or less numerous cells, and may therefore be 
styled a cell-community. It is clear that in such a case ad- 
vantageous adjustment to the surroundings is best secured on 
the principle of division of labour, by which the vital activities 
are shared among the members of the community. Evolution 
on these lines has resulted in the development of Digestive 
Organs, Respiratory Organs, Organs of Movement, &c., the 
complexity of which is very great in some of the higher groups 
of animals. Hence the need for some means of central control, 
some way of correlating the diverse parts of the body, and at 
the same time of adjusting the body to its environment. These 
duties are discharged by the Nervous System, with the aid of 
Sense Organs, which keep it in touch with external agents. 
The Sensitiveness and Spontaneity of a Metazoon, in fact, are 
more or less centred in the Nervous System and Sense Organs, 
and this is true to an increasing extent as we consider animals 
higher and higher in the scale. At the same time it must not 
be forgotten that every cell in the body is endowed with ad/ 
the primary properties of protoplasm, though cells specialize as 
it were in different directions, according to the nature of the 
organs of which they form a part. 
Nervous Systems or ZoopHyTES (C@LENTERATA). — The 
members of this primitive group, comprising Freshwater Polypes 
(Hydra), Hydroid Zoophytes, Jelly-Fish, Sea-Anemones, and 
