438 THE BIOLOGY OF HYDRA : 1961 



and is transported to the epidermis. I do not think that the hydra 

 cnidoblast is active in the process. 



FULTON: What do you find in the gastrovascular cavity? Are 

 these nematocysts or cnidoblasts? 



BURNETT: In the worm the nematocyst is naked, as I understand 

 it. In the hydra the nematocyst is always inside a cnidobkist. 



SLAUTTERBACK: We have sought an answer to that question 

 with the electron microscope and have never seen a nematocyst 

 in the tentacles of hydra which was not still within the cnidoblast 

 which produced it. It seems fair to assume that if the nematocyst mi- 

 grates it does so within its own cnidoblast. 



Also we have seen a cnidol^last migrating through the mesoglea 

 only once. It seems rather unlikely to me that very many cnidoblasts 

 can pass through the mesoglea and be repeatedly missed by us. 

 We have studied H. littoralis, P. oligactis and C. viridissima. 



BURNETT: This is interesting. H. pirordi appears to have a cnido- 

 blast migration pathway different from tliat of P. oligactis. I have 

 examined Dr. Brien's slides of H. pirardi and have observed that the 

 digestive cells in the base of the tentacle contain many ingested 

 nematocysts. Some of these nematocysts, in section, can even be 

 seen traversing the mesoglea towards the tentacle battery. 



SLAUTTERBACK: When nematocysts are seen in the gastro- 

 derm one must be very careful to determine that they are not de- 

 generating and that they are still within a living cnidoblast. 

 Many of these may appear to be variable even when examined 

 quite critically with the light microscope but at higher magnifica- 

 tions there are often signs of cytoplasmic degeneration in the cnido- 

 blasts and deterioration of the nematocysts themselves. 



BURNETT: Yes, we have made the same observation. However, 

 many of the cnidoblasts plus the enclosed nematocyst in the di- 

 gestive cells of the gastrodermis are normal. We have shown that 

 when a proximal portion of P. oligactis is grafted to the distal region 

 of H. pirardi, nematocysts characteristic of H. oligactis pass into the 

 enteron of H. pirardi where they are ingested. If this ingestion takes 

 place in the tentacles, the nematocysts of P. oligactis are passed 



