18 THE ELEMENTARY NERVOUS SYSTEM 



neurones, whose cell-bodies are situated in the integu- 

 ment instead of being gathered into special ganglia, ex- 

 tend, as in the vertebrates, from the skin to the central 

 nervous organs, the brain or the ventral ganglionic chain. 

 The motor neurones are essentially duplicates of those in 

 the vertebrates in that their cell-bodies lie within the cen- 

 tral organ whence their fibers extend to the appropriate 



FIG. 2. Diagram of the primary sensory s and motor m neurones of the ventral gan- 

 glionic chain of the earthworm showing their connections with the integument i and with the 

 muscles ms. (Modified from Retzius.) 



musculature. Intemuncial neurones are also abundantly 

 present in the earthworm, though their function here, in 

 contrast with that in the higher vertebrates, is pure nerv- 

 ous intercommunication, for it is very unlikely that the 

 earthworm possesses what in any strict sense of the word 

 can be called intelligence. Thus from a morphological 

 standpoint, the nervous systems of the higher annuals, 

 even including such forms as the earthworm, have much 

 in common, their three sets of interrelated neurones, 

 sensory, motor, and internuncial, being arranged upon 

 what is in the main a uniform plan. 



Considered from a physiological standpoint, the nerv- 

 ous system with its appended parts as just sketched falls 

 in the higher animals into three well-marked categories. 

 On the exterior of these animals are to be found sense 

 or receptors such as the free-nerve terminations 



