124 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The Nervous System. 
The nervous system of Penilia is composed of a supraesophageal 
ganglion and a series of ventral ganglia, but the supraesophageal and 
its commissures contain most of the nervous material of the adult. 
As the most important part of the nervous system, it is the first to 
develop. At a stage when the second antennae are plainly seen, 
represented by Fig. 20, the ectoderm is found to possess several 
layers on either side of the mid-line and at the anterior extremity of 
the embryo. These paired enlargements or accumulations of cells 
are the first beginning of the supraesophageal ganglion. The cells 
that compose them have no distinct walls, but in some sections 
appear almost circular, except where pressed against one another. 
The cytoplasm is light staining and the nuclei are very distinct. 
According to Samassa, the supraesophageal ganglion of Moina arises 
from paired rudiments in the same position as described for Penilia, 
but earlier in its development. Grobben found in Moina that at 
first the rudiment of the supraesophageal ganglion was single, but 
afterwards became paired. He found this unpaired rudiment to 
appear even earlier than Samassa, and the latter says in regard to 
this that Grobben was mistaken in the stage of his embryos in 
which he first found it. 
The ventral chain of ganglia also rise from paired thickenings of 
ectoderm. These appear externally as two slight ridges on either 
side of a groove, running down the mid-line on the ventral side of 
the embryo just anterior to the stomodaeum to the abdominal part 
of the embryo. Grobben calls this groove the “ primitivfurche ” 
in Moina. At a stage represented by Fig. 29, the cells composing 
these two folds have separated themselves into nine groups, which 
can be seen in longitudinal sections. The third one from the 
anterior end of the group is the smallest and least well marked. 
This is the ganglion of the second maxillar segment, and probably 
fuses with the one just anterior to it, as Grobben describes for 
Moina. This condition corresponds almost exactly with the figure 
given by Grobben for Moina, except in that case only eight ganglia 
are present, which is what we should expect, Moina having one 
pair less of thoracic aj^pendages. The supraesophageal commissure 
is seen in sections as a more or less definite string of cells running 
up to one side of the digestive tract. In the adult a cross-section 
