240 WM. A. KEPNER AND JOHN F. BARKER. 



there are few Hydras. When many Hydras are present it is 

 difficult to find Microstomas that lack nematocysts. When there 

 are few Hydras living with Microstoma in nature, specimens 

 that lack nematocysts are readily obtained. 



It has been found that by placing Microstoma in watch-glasses 

 containing fresh water and some fragments of water soaked, 

 dead leaves they can be kept indefinitely ; provided the air be 

 kept free of coal gas, illuminating gases, and formaldehyde 

 vapor. 



NEMATOCYSTS PASSING FROM ENDODERM INTO MESODERM. 



While this paper is not dealing with the methods by which 

 Microstoma handles nematocysts, it is of interest to record that 

 Kepner and Whitlock in 1917 observed a nematocyst being car- 

 ried down through the body of an endodermal cell and delivered 

 through the basement membrane into the mesenchyme. This was 

 a very slow process. The movement of the nematocyst could 

 be appreciated only when its position was recorded and then five 

 minutes later its position again observed. Its rate was not ac- 

 celerated as it passed through the basement membrane into the 

 mesoderm. The most remarkable feature of this was that the 

 obtuse end was directed towards the path along which the nema- 

 tocyst was being transported. 



NEMATOCYSTS UNIFORMLY DISTRIBUTED. 



Attention might be called to the additional fact that when these 

 nematocysts are taken up by the mesenchymal cnidophages they 

 are distributed uniformly at the epidermis by these attending 

 cells. So that immediately after a Microstoma has acquired a 

 supply of nematocysts at its surface, these nematocysts are uni- 

 formly distributed. It does not appear, however, that if the 

 anterior end, or any other region, lose its quota of nematocysts 

 that some will be taken from other regions of the body to take 

 the place of those lost. In this manner specimens sometimes 

 come to have a uniform distribution of nematocysts except for 

 some region (e.g. 'head") that is free from them. Despite 

 this fact, the subtle manner in which the cnidophages cooperate 

 to bring about a uniform distribution of the foreign nemato- 



