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 Amceba, 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 all 

 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 OF ZOOPHYTES (COZLENTERATA). - - The 

 members of this primitive group, comprising Freshwater Polypes 

 (Hydra), Hydroid Zoophytes, Jelly- Fish, Sea- Anemones, and 



