142 BULLETIN OF THE 



Distal to the coeciim, 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 comj^osed 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 vEolidse. Figure 13 rep- 

 resents a cross section through a cnidophore, showing the vacuolated 

 cells and the nematocysts therein {nVcy.). 



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'chi/.'). At a slightly later stage (Fig. 6) the mesen- 

 chyme has become greatly thickene(>, 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, IIL) the wall of the gastric diver- 

 ticulum (ga. dv.) has begun to protriide 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 crecum. 



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 figui-ed (Plate II. Fig. 9) shows a ceras whose length 

 is slightly greater than its diameter at tlie b^se. The mesoderm is 



