NERVOUS SYSTEM OF INVERTEBRATES. 



93 



how such an animal can have a common consciousness 

 (Fig. 68). 



4. Protozoa. — As these consist of a single cell and 

 not even a. cell aggregate, and therefore can have no 

 tissue of any kind, they 

 have no nervous system. 

 Yet there is a general sen- 

 sibility or response to 

 stimulus even in these lit- 

 tle spherules of proto- 

 plasm. All functions are 

 performed, though imper- 

 fectly, by these living 

 drops of jelly. This is 

 the starting point from 

 which by evolution have 

 been differentiated all the tissues and organs and func- 

 tions found in higher departments. 



Fig. 68. — Diagram of a medusa : n n, 

 nerve ; «z, mouth ; st, stomach ; rt, 

 radiating tubes. 



