MUSEUM OF COMPARATIVE ZOOLOGY. 143 



greatly flattened, and the entodermic wall of the coocum 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 coocum, constricting the lumen. The 

 cells of the distal compartment stain deeply, like those of the hepatic 

 coecum 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 tlirough 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 tlie 

 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, '92, 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 priori 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 chidophore 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 obtaili 

 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 



