23 
. lie ganglion and longitudinal stems had developed as principal cen- 
tres and conductors of nervous activity, still these appeared to a 
certain extent mere local thickenings of a continuous layer of nerv- 
ous tissue, which was either present outside of or else enclosed 
between the muscular layers of the bodywall. From this „nervous 
sheath,”” which is present in the two more primitively organized 
suborders of the Nemertines the muscular bodywall was directly 
innervated and I have suggested that in this equally distributed 
nervous stratum the conduction of nervous energy gradually be- 
came localized in distinet longitudinal and transverse tracts better 
adapted for that purpose, differentiating out of the primitive nerve- 
sheaih and thus giving rise to the transverse smaller branches me- 
tamerically springing from the longitudinal stems in the higher 
differentiated suborder: the Hoplonemertini. The nervous plexus 
itself I looked upon as corresponding to a very primitive iype of 
the distribution of nervous tissue in the body of the lower inverte- 
brates and I compared it to similar facts which ©. and R. HERT- 
wıgG had made known for the Actinia. The presence of this nerve- 
stratum in Nemertines has since been confirmed by DEWOLETZKY 
(Zoologischer Anzeiger n°. 62). 
In the Plathelminths, which have been so carefully investigated 
by A. Lan@ (20), the nervous system has histologically a great 
resemblance to that of the less differentiated Nemertines. More- 
over both the primary and secondary peripherie nerve-stems in 
the marine Dendrocoels were proved by this naturalist to be inti- 
mately connected together by an intricate system of very numerous 
commissures, interlacing with each other in all directions. No 
nervous stratum was found in these animals but it cannot be 
denied that a network of nervebranches so dense and complicated 
as the one discovered by Lane (former observers appear to have 
seen, but never to have recognized it as nervous tissue) is only a 
few steps in advance from a continuous nervous layer. 
And so all these investigations amongst the lower groups of 
the Invertebrates appear to point towards an increased com- 
plication of the commissural connections eulminating in the direct 
