ORGANIZATION OF PRIMITIVE CENTRAL NERVOUS SYSTEM 407 



Fig. 3. Dorsal view of the branching dendrites of motor neurons of the same 

 gangHon as in Fig. 1. Many of the neurons have dendrites which ramify over 

 large areas of the ganghon. The patterns shown in this figure and Fig. 1 are 

 typical of the general arrangement in arthropods and, to a less extent, of annelids. 

 Modified from Zawarzin (1924). 



of branching frequently still resembles that in nerve nets, where the neurons 

 appear to be non-addressed within each net. 



(4) In the simpler ganglia there are fewer classes of neurons, for example 

 only four have been recognized in the jellyfish ganglion (Horridge, 1956a). 

 Therefore fewer morphogenetic specifications are required to determine the 

 pattern of addressing, whether anatomical or physiological. On the other hand 

 in an arthropod, and to a lesser extent in a polychaete, many of the neurons 

 are unique and individually recognizable. 



(5) In particular, the number of types of efferent neuron is limited in the 

 simpler ganglia. That of the jellyfish has only two types; even in the relatively 

 complicated polychaete worm Nereis there are only about ten motoneurons 

 per ganglion on each side (Smith, 1957). With these ten, reduplicated along 

 the animal, it crawls and swims. This means that the simpler ganglia could 

 possibly operate by relying on the differential responsiveness of the dendrites 



