THE EVOLUTION OF THE NERVOUS SYSTEM 



783 



anemone (No. 2 of Fig. 201). In the jellyfish, which in contrast to the sea 

 anemone is a free moving animal, we find that the receptors are more highly 

 specialized and, therefore, much more sensitive, and that the impulses which 

 they receive are transmitted to a more definite nerve network, capable not 

 only of conveying the excitatory process from one part of the animal to an- 

 other, but also of imprinting on the impulse a characteristic rhythmic ac- 



Spencje 



2. 



Sea anemone 



3. 



9*14 



Simple form In 

 earthworm 



Addition of 

 association neurons 

 in earthworm 



Fig. 201. Diagram to show gradual evolution of nervous system from an epithelial cell (e) 

 and muscle fiber (m) in the sponge (/) to a specialized epithelial cell or receptor (r) and muscle 

 cell in the sea anemone (<?) ; then to a receptor and motor neuron joining in a ganglion (Gang.), 

 in simple form seen in the earthworm (3). Most of the ganglia in this and other segmented 

 invertebrates show also the internuncial or association neurons as indicated in 4.- 



tivity which brings about the contraction of the bell and the swimming move- 

 ment of the animal. The network now assumes the function of .an adjuster 

 as well as a transmitter of impulses. 



So far the adjuster is an extremely simple structure, and it is possible 

 that the effector and receptor organs are directly connected by fibers 

 running through it. When we come to the segmented invertebrates 



