202 THE BIOLOGY OF HYDRA : 1961 



ed hydra still discharge nematocysts when an Artemia strikes the 

 tentacle, but the nematocyst is quickly released from the tissues of 

 the hydra and the AHemia falls to the bottom of the culture dish. 

 If satiated with food, the hydra makes no effort to hold its prey. 

 Perhaps in your experiments, the nematocysts discharged but were 

 not retained by the cnidoblasts. 



ROSS: I only refer to the original observation which was that 

 when an animal is on a shell you can brush another shell across it 

 and there is not the faintest sign of a response. The tentacles are 

 just brushed aside; they don't stick to it in any way. Yet you have 

 this phenomenal behavior which is elicited when the animal is off 

 a shell; it practically pounces on the test shell. Starting from that 

 observation, we went on and did this other experiment. I wouldn't 

 say that this is the complete answer, but I think it raises the whole 

 question of nematocyst control \ ery sharply, even though more in- 

 vestigation is required to clear it up. You certainly have a very 

 different type of behavior depending whether the animal's foot is 

 on a shell or not. It seems to us that this bcha\ ior, when it occurs, 

 begins with nematocyst discharge. 



LOOM IS: Do you think it might be a matter of the shell trans- 

 mitting calcium to the tentacle and making it sticky? 



ROSS: I've tried a good many models of shells and also shells 

 boiled in alkali to remove organic material. The anemone does not 

 respond to these; cleaning the shells destroys the activity. If you 

 present CaUiactis with a perfect plaster of Paris model of the shell, 

 it shows no interest. The rest of the story (I haven't time for the 

 evidence here ) is that some substance in the mollusc shell, and not 

 derived from the crab but from the mollusc, triggers the nematocyst 

 discharge and the subsequent behavior pattern. It is not responding 

 to the calcium of the shell, or to any other inorganic constituent, or 

 the characteristically sculptured surface of the shell. In fact, the 

 anemone gets on the shell occupied by the hermit-crab by respond- 

 ing to the ghost of the long-dead mollusc that used to live there. It 

 has nothing to do with the crab as such. 



SLAUTTERRACK: Does anyone care to go into metaphysics 

 further? If not, I declare this meeting adjourned. 



