438 jJournaL or Comparative NEuROLOGY. 
deeper position than does the nervous system of most Poly- 
cheta. Itis separated from the hypodermis by the circular 
muscles and is enveloped by an elaborate protective tissue. 
The protective tissue is of two kinds: an inner spongy 
layer, the neuroglia, which is of ectodermal origin, and the 
neurilemma, which is mesodermal in origin. 
The mushroom bodies of insects and decapod crustacea 
are represented in the brain of Vevezs by the anterior masses of 
small nuclei. 
In some species of Nereis the optic ganglion lies beneath 
the anterior eye, in others it lies within the brain capsule. 
There is no neuropil in the nerve cord. 
There is one small median and two larger lateral connec- 
tives between each two successive ganglia of the ventral nerve 
cord. 
The sheaths of the nerve fibers have no nuclei; hence they 
must be a product of the fibers themselves. 
The nerve cells of the ventral cord commonly have one or 
more centrosomes. 
The giant fibers are nervous in function and are connected 
to the periphery by ordinary centrifugal fibers. The giant fibers 
do not give off fibrillations, and nervous relation with other 
fibers is established directly between the axis cylinders. 
Where certain decussating fibers cross, their axis cylinders 
are united by anastomoses. 
The ends of certain centripctal fibers of the same set are 
always united by anastomoses. 
Contact between their axis-cylinders may possibly be one 
of the means of bringing nerve fibers into functional relation 
with each other. 
MATERIAL. 
In this study the following material has been used: the 
common crayfish (Cambarus sp ?), Limulus, Branchipus, Gam- 
marus, Nerets, Polynoe, Lepidonotus, and the common earth 
worm. 
es 
