Influence of the Nervous System 695 
worms began to disappear below the surface. Examination 
showed that in every case a head with a functional mouth had 
been regenerated. About 250 worms were operated upon. ‘The 
mortality was low; in some series only from I to 5 per cent died. 
Of the survivors about one-half regenerated heads. It should be 
borne in mind that in order to obtain the early stages in the changes 
of the injured regions, a large number of worms had to be pre- 
served long before it could be determined whether a head would 
or would not regenerate. 
According to Retzius the nervous system in the earthworm 
comprises two quite dissociated parts. ‘lhe motor cells are found 
exclusively in the cord, the sensory cells in the skin. ‘The cord cells 
are of two general types; the great majority are unipolar cells the 
processes of which soon bifurcate, the branches passing into the 
next anterior or posterior segment or to the other side of the same 
segment. ‘There is a second type of cells usually very much larger, 
with its branching neurones passing in all directions but rarely 
beyond the limits of the segment. It is thus seen that the motor 
nerves in any segment take their origin in the same segment orin the 
next anterior or posterior one. ‘The sensory cells send their nerve 
fibers into that part of the cord within the same segment. In 
order to remove from an amputated end all the motor stimuli the 
cord must be completely removed from at least 3 segments adjoin- 
ing the cut surface. Concerning the giant fibers, so called, and 
their functions, practically nothing is known. Even their nervous 
nature is questioned. ‘They are believed (Clarapede) to take their 
origin near the anterior end of the nerve cord. As these “ fibers”’ 
are intimately bound up with the cord, they were removd with the 
rest of the cord from the amputated end. 
Table 4 gives a list of individuals taken from one series (C); 
the number of segments amputated, the length of cord removed, 
the time, the number of segments regenerated, etc., are also given. 
The rate of regeneration was extremely variable, and apparently 
independent of the number of segments amputated and the length of 
cord removed. There were of course other injuries particularly 
to the mesenteries, nephridia and sometimes the inner muscle 
layer. It is possible that the rate of regeneration was largely 
