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- 
Seater ateed brates, of which our knowledge is considerable, 
Neuron, much enlarged it is usual for a nerve-cell to be prolonged 
N., Nucleus; .F., nerve- . ° . 
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 xeuyon, 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 ¢o 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. 
