No. 415.] NEREIS VIRENS SARS. 569 
unipolar ganglion cells. The ganglionic masses can be fol- 
lowed continuously through three metameres. Four pairs of 
large nerves are given off from this portion of the cord to the 
supernumerary parapodia and the body wall. This ganglion 
measured 0.6 mm. through its — 
widest part, which is 0.2 mm. 
more than the greatest diameter 
of any normal ganglion from the 
same region. There is thus an 
evident centralization of nerve 
substance at the anterior end of 
the abnormal cord, comparable 
perhaps to the centralization 
Fic. 6. — Anterior face of a transverse section 
through the seventh abnormal ganglion ; 
found in the subcesophageal showing neuropil, ganglion cells, and a 
60.) 
" pair of parapodial nerves. (X 
ganglion of the normal nervous 
system. Throughout the remainder of the interpolated nerve 
chain there are two longitudinal connectives with a ganglionic 
enlargement in each of the twelve succeeding metameres. Fig. 6 
is a transverse section through one of these ganglia. The con- 
nectives and ganglia are small, taper gradually at the posterior 
end, and terminate independently. Three of the ganglia aver- 
aged 280 p in greatest diameter, while the longitudinal con- 
nectives varied from 300 p, at the anterior end of the chain, 
to 125 w, near the posterior end. The normal connectives and 
ganglia of the same region measured respectively 300 # and 
400 win diameter. Five pairs of nerves are given off latero- 
ventrally from each ganglion and connectives to the adjacent 
metameres. This agrees with the number of pairs innervating 
each normal somite of the worm. In addition to their small 
size the connectives present another peculiarity in the absence 
of the three giant fibres, which are such conspicuous structures 
in the normal cord. 
From the above description of the animal we find (1) that 
the abnormality is serial in its occurrence, makes its appear- 
ance abruptly, and extends through twenty-three somites ; (2) 
interpolated in each of the first twelve abnormal metameres is 
a pair of appendages, together with all the organs which, in 
the normal animal, are included ventrally between the parapodia; 
