NEMATOCYSTS OF MICROSTOMA. 273 



The question naturally presents itself as to what kind of cell 

 it is that has thus entered the vacuole. In all favorable nuclei 

 that are found within the vacuole there is present a deeply 

 staining nucleolus about which there is to be seen a vacuole 

 (Figs. 8 and 10). This makes the nuclei more closely resemble 

 those of the amcebocytes than those of the other mesodermal cells 

 of the Microstoma. So I conclude that in each case an amcebo- 

 cyte enters the vacuole to take charge of the nematocyst as do 

 the cnidophages of the seolids. 



It is interesting to observe how these nematocysts are handled 

 by the invading cell of each vacuole. 1 The nematocysts when 

 they lie alone within the vacuoles are pointing indifferently 

 in all directions, but after they have been taken charge of by the 

 cnidophages they have in all instances their discharging poles 

 directed towards the exterior of the body. Martin also observed 

 this final exact orientation of the nematocysts. He says of it 

 "the large barbed nematocysts in their final position, always lie 

 so that the thread, when it is discharged, will pass out of the an- 

 imal, although they may lie pointing in any direction while they 

 are still in the gut cells or the body-cavity. This rule does not 

 seem to hold good in the small cylindrical nematocysts, which, 

 as far as I can see usually lie almost parallel to the surface. It 

 is very difficult to say how such an orientation can be effected, 

 but something of the same kind has been detected in ceolids, and 

 I believe that the same difficulty is present in the nematocysts 

 of the tentacles in coelenterates " (Martin, 1908, p. 267). It 

 would indeed be a most difficult matter to understand their 

 orientation if Hallez and Martin are correct in describing these 

 exotic nematocysts as always lying within vacuoles. The cni- 

 dophages appear to be the vital agents which properly orient 

 the nematocysts. 



In my preparation one barbed nematocyst was observed that 

 had been apparently inaccurately discharged. This was taken 

 from near the middle of a specimen that when fixed remained 

 extended. In such distended specimens the mesoderm near the 

 middle of the animal is reduced to a very thin layer; this together 



1 Hereafter we shall speak of this invading cell of the vacuole as a 

 "cnidophage." 



