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- t 



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 



i i* rr i 1 Fig- 1008. A, Stages in the 



complex adjustments to effect with the en- Evolution of the Neurons of a 

 vironment than their fixed relatives, possess, J f^^^- m \ L"- 

 as might be anticipated, a more centralized Anemone, 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 it is largely superseded by the central nervous system, which 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 



