470 



COMPARATIVE ANATOMY 



In some coelenterates, the primitive ganglion cells form a loose subcu- 

 taneous network or plexus in which, as experiments show, nervous impulses 

 may be carried in any direction. Morphologists incline to the opinion 

 that the nerve net of coelenterates becomes the central nervous system of 

 higher animals. 



A final step in the evolution of the neuron is taken by the flatworms, 

 in which the neurons, like the neurosensory cells of coelenterates, trans- 



NEUROSENSORY CELL 

 I BIPOLAR NEURONE 



UNIPOLAR NEURONE 



Fig. 387. — A diagram illustrating hypothetical stages in the phylogenesis of the 

 characteristic sensory (afferent) neuron of vertebrates. Earlier stages at the left, final 

 stage at the right. Arrows suggest that the direction of growth of the neurite is away 

 from the source of stimulus. The diagram assumes that the primitive neurosensory cell 

 becomes the definitive sensory neuron and that the definitive sense receptor is secondary. 

 It is quite possible, however, that the neuron is secondary and that the neurosensory 

 cell becomes the definitive receptor. (Redrawn after Ariens-Kappers modified.) 



mit nervous impulses in only one direction. In contrast with neuro- 

 sensory cells, however, a neuron has at least two nervous processes, of 

 which the dendrite carries impulses toward the cell body, while the 

 neurite (neuraxon) carries impulses away from the cell body. Each 

 neuron has but one neurite, and may have one or many dendrites. In 

 ontogenesis the neurite grows away from the source of stimulation and 

 the dendrites towards the stimulus. 



