142 BULLETIN OF THE 
Distal to the coecum, at the apex of the ceras, lies a sac which in the 
adult opens to the exterior and is connected at its proximal end with 
the coecum in such a manner that the walls of the two organs are con- 
tinuous, and their cavities confluent through a communicating canal. 
The existence of this communicating canal, although in past times called 
in doubt, has recently been correctly reaffirmed by Herdman (’90, p. 52). 
The walls of the sac are composed of almost completely vacuolated cells, 
which contain nettling organs or nematocysts. Since the cells of such 
sacs contain nematocysts, the sacs are called nematophores or cnido- 
phores. Cnidophores are characteristic of the Aolide. Figure 13 rep- 
resents a cross section through a cnidophore, showing the vacuolated 
cells and the nematocysts therein (nt’cy.). 
The first indication of the formation of a new ceras is a thickening of 
the mesenchyme at the base of a young ceras and upon its ventral aspect 
(Plate I. Fig. 5, ms’chy.!). Ataslightly later stage (Fig. 6) the mesen- 
chyme has become greatly thickened, and a protuberance of the ectoderm 
has occurred. Karyokinetic figures indicate that the mesenchymatous 
mass is growing by cell proliferation. The growing mesenchymatous 
cells, as well as the adjacent cells of the gastric diverticulum, stain 
more deeply than those of other regions. 
At a slightly later stage (Fig. 7, II.) the wall of the gastric diver- 
ticulum (ga. dv.) has begun to protrude into the thickened mesenchyme, 
and the ectoderm is sharply evaginated. Still later (Fig. 8, IV.) these 
features become more pronounced. The mesenchymatous cells become 
arranged into three or four layers, of which one is closely applied to the 
ectoderm, and another to the hepatic ccecum. 
It is important to note, (1) that the ectoderm of the new ceras lies, at 
at an early stage, in the angle made by the body wall with the ventral 
wall of the next older ceras; (2) that the mesenchyme is directly con- 
tinuous with that of the ceras, and is in fact transitional from the meso- 
derm of the trunk to that of the ceras; and (3) that the budding 
entoderm lies at the distal extremity of the gastric diverticulum, that 
is to say, at the angle made by the gastric diverticulum, and the last 
ceras arisen therefrom (Fig. 3, cer. 3). The mother cells of each of the 
three layers of the incipient ceras thus lie in the outer margin of the 
region whence the corresponding layer of the last formed ceras has 
arisen. With the development of the ceras it gradually becomes farther 
removed from the next older one (Fig. 3). 
The next stage figured (Plate IT. Fig. 9) shows a ceras whose length 
is slightly greater than its diameter at the base. The mesoderm is 
