NERVE CELLS 183 



of cells which are otherwise undifferentiated and exposed at the 

 surface. Here, then, the stimulus received by one part of a cell 

 is transmitted to another part of the same cell. In other 

 Ccelenterates, some of these epithelial 

 cells are prolonged at the deeper ex- 

 tremity into slender fibres, which are 

 regarded as nerve fibres instead of 

 muscle fibres. In this case the en- 

 tire cell is nervous in function, it 

 receives and transmits stimuli. 

 Again, cells are found which do not 

 reach to the surface of the epithelial F IG - ^--Diagram of nerve 



cells found beneath the ecto- 



layer to which they belong, and are, derm of a jelly-fish, 

 therefore, probably not sensory. 



These cells, however, are multipolar nerve cells. They serve 

 only to transmit stimuli. We have, therefore, three types of 

 nerve cells: 



1. Sensory Transmitting Motor. 



2. Sensory Transmitting. 



3. Transmitting. 

 While in the taste buds are found: 



4. Sensory. 



436. The first class is an extremely low type of differentiation, 

 and is not found in the sensory-motor mechanism of the higher 

 animals. 



437. In the jelly-fish many cells of type three are found, 

 and they are largely grouped in a circle of ganglia around the 

 edge of the umbrella. This circle of ganglia constitutes a simple 

 central nervous system, arranged with reference to the radial 

 organization of the animal. 



438. In Annelids, the central nervous system is bilaterally 

 arranged. There is a double ganglion in each segment, and all 

 the ganglia are connected longitudinally by a double nerve. 



