NERVOUS SYSTEMS OF INVERTEBRATES 7 
Although neurons belong to the external cell-layer they do 
not remain at the surface of the body, but sink as it were more 
or less inwards.to take up a more sheltered position, leaving to 
sense-cells the duty of reception of stimuli from external agents. 
In some Jelly-Fish we find cells of the external layer which are 
beginning to sink in to constitute neurons, while others have 
actually done so and acquired at the same time a more specialized 
shape (fig. 1008, a). 
A nervous system of very primitive kind is found in a Sea- 
Anemone, which, leading as it does a sluggish life fixed to some 
firm object, does not require any very ela- 
borate correlating mechanism. There is here 
a delicate continuous nerve-layer underlying 
the ectodermal cells that directly adjoin the 
exterior, and made up of innumerable neurons 
of which the extensions run in all directions 
(fig. 1008, B). Even in this case, however, 
there is a certain amount of centralization, 
for the nerve-layer is thicker in the upper side 
of the body where the mouth is placed, and 
in the tentacles which fringe this region. 
The free-swimming Jelly-Fish, having more 
* ‘ Fig. 1008.—a, Stages in the 
complex adjustments to effect with the eM-  Bwolution of the Neurons of a 
; : lly-Fish, enlarged. 1, Part 
vironment than their fixed relatives, possess, JP Ninctavcr in a Sex 
as might be anticipated, a more centralized a showing Neurons, en- 
nervous system. It is true that there is here 
also a continuous nerve-layer in the deeper part of the ectoderm, 
but part of this is concentrated into what may be called a central 
nervous system. This may be either in the form of a double 
nerve-ring placed near the edge of the umbrella, or it may con- 
sist of small masses of neurons placed at regular intervals in the 
same region. 
Nervous SysTEMS oF SEGMENTED Worms (ANNELIDA).—The 
members of this group are comparatively complex in structure, 
and possess a well-defined nervous system, that conforms to 
the two-sided or bilateral symmetry of the body. The primi- 
tive nerve-layer in the ectoderm is retained more or less, but 
it is largely superseded by the central nervous system, which 
consists of a nerve-ring surrounding the front end of the diges- 
tive tube, and a double nerve-cord running along near the under 
