NEMATOCYSTS OF MICROSTOMA. 



241 



cysts is remarkable and lends weight to the inference that the 

 nematocysts are being collected by the Microstoma to be used. 



NEMATOCYSTS RETAINED INDEFINITELY. 



Again if the nematocysts of Hydra are carried out to the 

 surface of Microstoma by way of eliminating them as indigestible 

 foreign bodies, the question arises as to why they are held so 

 long at the surface. Within twelve hours after a Hydra has 

 been ingested by Microstoma, the nematocysts are distributed 

 over the rhabdocoele's body. The process of getting these ob- 

 jects to the surface of the flatworm is, therefore, a matter of 

 about twelve hours. When, however, they arrive at the surface 

 they are held there indefinitely. Even when animals are kept 

 under adverse conditions in the laboratory and they show more 

 and more conspicuous decline, the nematocysts are yet retained. 

 Specimens have been seen growing weak and beginning to 

 rupture or break up and yet retain the nematocysts over the 

 epidermis that remained intact. This retention of the nema- 

 tocysts, likewise, suggests that they have been collected for use. 



AN EXPENSIVE METHOD. 



Again this is an expensive method of dealing with objects that 

 could be thrown out at the mouth as are other indigestible 

 materials. For in this process much energy must be spent by 

 the endodermal cells and cnidophages in handling the nemato- 

 cysts; and, further, when the nematocysts are discharged they 

 carry with them their attending cnidophages. The cnidophages, 

 therefore, in attending the nematocysts act against their own 

 welfare in that in the end they lose their own lives. 



MICROSTOMA INCURS A DANGER. 



Even the Microstoma as a whole feeds upon Hydra at a risk. 

 For sometimes Hydra turns upon Microstoma and eats it. 

 Hydra, however, seems to have difficulty in ingesting Mic- 

 rostoma as the following observation of January 18, 1917, shows. 

 The Microstoma at once played along the side of Hydra viridis 

 that had been placed with it. The worm passed to and fro along 

 the surface of the polyp's body. Twice it came to rest amidst 



