NEMATOCYSTS OF MICROSTOMA. 249 



Sometimes these collisions caused the one animal to glide down 

 along the side of the other ; at other times the collisions would 

 be head on. In each case the collisions were so evident as to 

 cause a shunting of one or the other specimen. At 11.30 

 A.M. while Microstoma lay quiet, Stenostflma made a con- 

 tact with Microstoma and stroked the entire left side of 

 the Microstoma with its right ciliated pit. When Stenos- 

 toma's head had come to be in contact with the posterior 

 end of Microstoma, the latter discharged two nematocysts 

 into the region of Stenostoma's right ciliated pit. At once 

 a rupture took place in the Stenostoma's epidermis and from 

 this wound granular material oozed. Despite this breaking of 

 the Stenostoma's body in the region in which it had been stung, 

 the Stenostoma was anchored to Microstoma by the nematocysts, 

 the poison sacs of which were yet held within the Microstoma. 

 The two animals were anchored thus long enough for me to call 

 Dr. I. F. Lewis to my side and make a demonstration of the 

 situation to him. Soon after this demonstration was made, the 

 two poison sacs left the Microstoma's body and the latter swam 

 away. The Stenostoma, which up to the time it was stung had 

 been incessantly active, lay quite inactive for ten minutes with 

 two nematocysts hanging from the wound 011 the right side of 

 its anterior end. After that it gradually recovered itself. 



It has thus been demonstrated that Microstoma uses the nem- 

 atocysts which it takes from the Hydra. 



SUMMARY. 



1. Microstoma manipulates the nematocysts of Hydra either 

 (a) as a means of eliminating indigestible parts of its food, or 

 (&) a means of defending itself. 



2. The fact that the nematocysts are distributed uniformly 

 over the surface of Microstoma within 12 hours after a Hydra 

 has been ingested, suggests that this is not a process of elimina- 

 tion, (a), but is done in order that the nematocysts may be used, 



3. The fact that the nematocysts are retained indefinitely 

 suggests that they have been taken up and retained for use. 



