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THE POPULAR SCIENCE MONTHLY 



pile, fibrillating as they pass to terminate as motor nerve-fibers in the 

 muscles of the adjacent part of the body. These cells with their proc- 

 esses constitute the primary motor neurones of the earthworm and, like 

 the sensory neurones, they may be present in any one of the three nerves 

 of a segment. Their longitudinal extent is probably not much beyond 

 a single segment. 



The primary sensory and motor neurones not only give rise to the 

 nerves of the earthworm, but they contribute a larger part of the sub- 



Fig. 2. Transverse Section of the Ventral Nervous Chain and surrounding 

 Structures of an Earthworm, cm, circular muscles ; ep, epidermis ; lm, longitudinal 

 muscles ; mc, motor cell-body ; mf, motor nerve-fiber ; sc, sensory cell-body ; sf, sensory 

 nerve-fiber ; vg, ventral ganglion. 



stance of each ganglion. As stated in the first article, they form when 

 together the necessary elements for the simplest, conventional reflex-arc. 

 How they are related to one another in the neuropile is not conclusively 

 settled, but, judging from the work of Apathy (1897) and others, the 

 connection here as in the nervous net is one of direct continuity. 



Besides the motor and sensory neurones, the central nervous organs 

 of the earthworm contain a considerable number of so-called association 

 neurones. These are nerve-cells with longer or shorter processes that 

 connect parts within the same ganglion or run from one ganglion to 

 another. They give rise to no fibers that extend into the nerves and 

 hence they are strictly limited to the central nervous organs. Their 

 longitudinal extent is seldom over more than one or two segments. 



Since the sensory, motor, and association neurones thus far described 

 make up the bulk of the ventral nerve-cord of the earthworm and since 

 none of these have a longitudinal extent of more than a few segments, 

 it follows that the cord must be conceived as made up of an immense 

 number of overlapping short neurones which in this collective way 

 stretch over its hundred and twenty or more segments. But the nerve- 

 cord of the earthworm is not composed exclusively of short neurones. 

 In its dorsal portion are three giant fibers which, though their nature 

 has been even recently disputed, are without much doubt nervous 



