MUSEUM OF COMPARATIVE ZOOLOGY. 143 
greatly flattened, and the entodermic wall of the coxcum is composed of 
cuboidal deeply stainable cells. At a stage at which the length of the 
ceras is less than twice its greatest diameter (Fig. 10), a circular fold 
occurs in the distal third of the cecum, constricting the lumen. The 
cells of the distal compartment stain deeply, like those of the hepatic 
cecum of the preceding stage. They do not at all resemble the un- 
stained cells of the ectoderm. 
At a stage only very slightly older than the preceding (Fig. 11), one 
finds the cells of the distal sac partly vacuolated and containing nema- 
tocysts. This distal sac can be traced forward into the oldest stages, 
and it becomes the cnidophore. Its lumen constantly retains its con- 
nection with that of the hepatic coecum through the communicating 
canal. A communication with the outside world at the apex of the 
ceras is established only at a later stage by a close approximation of the 
cnidophore to the apex of the ceras and a disintegration of the apical 
ectodermal cells. 
Herdman asserts in two or three places (90, p. 52, and Herdman 
and Clubb, ’89, p. 233, 792, p. 552), that the cnidophores arise by an 
invagination of the ectoderm on the apex of the ceras; but although in 
his last paper he says he has “shown” it in the earlier ones, I fail to 
find that he has offered the slightest evidence for his statement. It 
seemed indeed a priort more probable—from what we know of the 
origin of protective organs, and especially of the origin of nematocysts 
in the Cnidaria—that the cells of the cnidophore had an ectodermal 
origin. But they have not. This is the conclusion to which I am forced 
by the following considerations. (1.) The development just outlined, 
which has been traced in a series so complete as to leave little chance 
for misinterpretation. (2.) The absence of an external opening until 
quite a late stage. Since the axis of the cnidophore does not in later 
stages coincide with that of the ceras, and since it is not easy to obtain 
sections which pass through the entire axis of the ceras, especial care 
must be exercised in determining the absence or presence of an apical 
opening. Figure 12 is a strictly axial section. The apical ectodermal 
cell shows signs of degeneration, and its outer surface is sunken in. 
(3.) The presence of nematocysts in those cells also which lie in the 
hepatic coecum proximal to the constriction, and for which no one has 
maintained a derivation from an invagination of the apical ectoderm. 
The cells of the hepatic coecum, especially at a late stage, show large 
numbers of nematocysts of the two kinds mentioned by Herdman (’90). 
Cf. Figures 14 and 15. That these nematocysts have been developed 
