ALLISON L. BURNETT 437 



growth would serve only to push the nematocysts towards the base 

 of the animal. 



CROWELL: If you cut the tentacles off, nematocysts first form 

 in the column, and then move into the tentacles from the lower 

 region. How do they get there? 



BURNETT: They are simply forced distally by an active cell 

 proliferation in the growth region of the hypostome. 



CROWELL: No, I'm not talking about the ones that grow nor- 

 mally, but when one cuts the tentacles off and mobilizes nematocyst 

 formation elsewhere, how do these cnidoblasts move? 



BURNETT: The answer is still the same. Cnidoblasts of a newly 

 regenerated tentacle are pushed there by sub-hypostomal growth 

 processes. Some cnidoblasts are pushed into the peduncle. Others 

 pass into the enteron where they are swept to the tentacles; 

 many are ingested by digesti\e cells in this area. However, in 

 Pelmatohyclm oligactis these ingested cnidoblasts will be trans- 

 ferred to an adjacent tentacle battery only if that battery's sup- 

 ply of nematocysts is depleted. 



CROWELL: They are transported in the gastro-vascular cavity? 



BURNETT: That's right. 



LOOMIS: There is another piece of evidence suggesting that nem- 

 atocysts reach the tentacles through the gastro-vascular cavity. This 

 is the well-known fact that when the worm Micwstomiim eats 

 hydra, the undischarged nematocysts migrate through the worm's 

 tissues until they are in position in the ectoderm and ready to be 

 used by him. In other words, if nematocysts can travel through the 

 endoderm to the ectoderm of a flatworm, then almost certainly they 

 can do the same through the tissues of a hydra. This would explain 

 how a hydra can arm its tentacles within twenty four hours after 

 being fed e\'en though it takes about a week for new cells to grow 

 out from the hypostome to the end of a tentacle. 



BURNETT: Also, I believe that the nematocysts in the worm 

 Microstomum are moved passively through the tissues of the worm. 

 I think the nematocyst is phagocytized by a worm mesodermal cell 



