IsEMATOCYSTS OF MICROSTOMA. 275 



There would be no protection gained by the flatworm in 

 passing them into the tissue of its body if in the digestive fluids 

 of Microstoma the nematocysts are likewise not discharged; 

 and in case they did discharge within the digestive fluids of Mi- 

 crostoma they would have done their injury before they had been 

 taken up by the tissues of the Rhabdoccele. 



Glaser (1909) has further demonstrated that when nemato- 

 cysts pass from digestive fluids into fresh water in all cases they 

 discharge. Here then are seen grounds for making the inference 

 that his careful handling of the nematocysts is done in order to 

 prevent their haphazard discharge in the region of the mouth 

 as they are egested. Over against this inference is the fact that 

 in aeolids certain nematocysts pass from the alimentary canal 

 with the waste food while others are more carefully handled and 

 discharged by the cnidophages of the cerata (Glaser, 1909): 

 also w r hen a flatworm is bold enough to attack a living Hydra 

 with its anterior end it can hardly be considered to be so serious 

 a matter to have nematocysts discharged casually within the 

 vicinity of the mouth. 



Again the end may be to avoid discharge of nematocysts by 

 means of distortion within the enteron; for Glaser (1909) has 

 shown that nematocysts when distorted do discharge. This 

 suggestion I think can be dismissed with the inference that within 

 the mesoderm the nematocysts are just as liable to distortion 

 as within the enteron; for through the muscular actions of the 

 body the ova are seen at times to suffer considerable distortion. 



The third suggestion is that these bodies are taken up to be 

 used. Grosvenor says: ' 'No one can have witnessed the re- 

 action of an seolid to various stimuli . . . without being convinced 

 that the cerata are used as a means of defence' ' (Glaser, 1910, 

 p. 138). Likewise no one can have witnessed the discharge of 

 nematocysts of Microstoma when stimulated by pressure or by 

 acetic acid without looking upon them as organs of defense. 



It taxes my faith seriously to believe that endodermal cells 

 select nematocysts for so remote a purpose as .that of defending 

 the cell-colony at the ectodermal surface, and yet until further 

 evidence can be obtained, if I must take a stand, I conclude that 

 these exotic nematocysts are handled by Microstoma for purposes 

 of defense. 



