432 WILSON. [Vot. Ill. 
tinuation with the foundations of the cesophageal commissures 
and of the ventral chain. As regards the first point, Kleinen- 
berg’s failure to observe the double foundation of the cephalic 
ganglia seems to have been due simply to his not having studied 
sufficiently early stages; for the earliest stage he figures is 
already as far advanced as that shown in Fig. 79 of this paper, 
in which the two ganglia have already fused. 
On anatomical grounds there is considerable reason to believe 
that the cephalic ganglionic mass was primitively unpaired, as 
it still remains in Protodyri/us, and that its bilobed character in 
the higher forms is the result of the bilateral differentiation of 
the head (paired sense-organs, etc.). Hatschek endeavored to 
show that a median unpaired foundation (Scheitelplatte) for the 
cephalic ganglion was characteristic of annelidan development 
generally (as typically shown in the development of Polygordius), 
and that this apical plate represented the central nervous system 
of the ancestral Trochozodn. A comparison of the actual 
development of the various forms, however, not only shows a 
marked divergence between the foetal and larval types, which 
might have been expected, but also indicates that both types 
must be more carefully examined before any trustworthy conclu- 
sion can be reached in regard to the apical plate. As regards the 
foetal types the evidence is conflicting, but on the whole appears 
at present to indicate a double foundation for the cephalic 
ganglia. The facts in Lumbricus seem to admit of no doubt, 
and agree with Kennel’s observations on Ctenodrilus (No. 26) — 
a form which possesses a very primitive nervous system. In the 
case of Criodrilus, as Bergh has pointed out (No. 5), Hatschek’s 
earliest figures (No. 18) show the apical plate as a distinctly 
bilobed structure, though he describes it in the text as median 
and unpaired. Vejdovsky positively asserts the unpaired char- 
acter of the apical plate in Rhynchelmis (No. 44); but, as he 
expressly states, he did not observe the apical plate until it had 
become a large mass, entirely filling the cephalic cavity, and we 
have seen that in Lwsmbricus the double character of the founda- 
tion is only manifest at a much earlier period. 
As regards the larval types, all observers agree that the 
foundation of the cephalic ganglia arises in connection with an 
unpaired sense-organ (“ Sinnesplatte’’), which is usually median, 
but sometimes asymmetrical (Kleinenberg, Salensky). The 
