6 NERVOUS SYSTEM AND SENSE-ORGANS 



Corals, correspond to a fairly early stage in the evolution of the 

 Metazoa from simple colonies of Protozoa, and furnish us with 

 some idea of the way in which nervous systems have arisen. 

 Reduced to its simplest terms the body of such an animal is 

 practically a living stomach, and is made up of two layers of 

 cells an inner one (endoderm) and an outer one (ectoderm). 

 We are here more especially concerned with the outer layer. It 

 would appear that the nervous system^was first evolved in the 

 interest of adjustment to the surroundings, and it is not there- 

 fore surprising to find that it has come into existence by modi- 

 fication of some of the cells making up the ectoderm, since this 

 layer immediately adjoins the outer world. The 

 same is also true for the essential parts of all 

 the sense-organs. These specialized nervous 

 elements are known as nerve-cells or ganglion 

 cells, which constitute so many centres of cor- 

 relation and control. A typical cell of the kind 

 is star -shaped and possesses a large nucleus, 

 but it may also be round or ovoid. In Verte- 

 brates, of which our knowledge is considerable, 



Fig. 1007. -A Typical . . 



Neuron, much enlarged, it is usual tor a nerve-cell to be prolonged 



N., Nucleus; N.F., nerve- . , r i 1 1 



fibre (cut short). into a number of branching prolongations, 



and one nerve -fibre, which may be of very 

 great length (fig. 1007). It is convenient to speak of the cell 

 with its extensions as a neuron, and investigation will probably 

 show that the neurons of Invertebrates are broadly similar to 

 those of Vertebrates, though in many instances our knowledge 

 is here very incomplete. It is clear that the nerve-cells need to 

 be in communication with the environment, the parts which they 

 control or correlate, and other similar cells with which they co- 

 operate. This is provided for by the slender extensions of the 

 cell-body, which constitute lines of communication. There is 

 reason to think that the branching extensions are paths of con- 

 duction to the cell, while the nerve fibre is a similar path from 

 the cell. It was formerly believed that the neurons in a nervous 

 system are united together by their processes into a complicated 

 net-work or plexus, but it is now known that in Vertebrates at 

 any rate this is not so, though the slender extensions of one 

 neuron are closely adjacent to those of others. The same thing 

 is probably usually true for Invertebrates. 



