THE EVOLUTION OF THE NERVOUS SYSTEM 



783 



point on the surface, as, for example, in the case of the sea 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 special- 

 ize 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 another, 



Sponge 



2. 



Sea anemone 



Simple form in 

 earthworm 



Addition of 

 association neurons 

 in earthworm 



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

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

 cell in the sea anemone (.2) ; then to a receptor and motor neuron joining in a ganglion (Gang.), 

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

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



but also of imprinting on the impulse a characteristic rhythmic activity 

 which brings about the contraction of the bell and the swimming movement 

 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 



