Excitation of Nematocysts

Abstract

THERE has in the past been considerable discussion concerning the nature of the stimulus which causes the nematocysts of Cœlenterates to discharge, In some cases, it is quite clear that the mechanical stimulus to the cnidoblast causes an, explosion of the enclosed nematocyst. But for the majority of nematocysts, discharge only takes place in the presence of food, and it has been suggested that the stimulus is essentially a chemical one. During a long series of experiments on the feeding of Anemonia sulcata, interesting evidence has been obtained as to the mode of excitation of nematocysts. In agreement with Parker and Van Alstyne¹, I find the cnidoblasts of the tentacles of Anemonia are independent effectors. They respond, however, selectively to food. If touched by a clean glass rod, no discharge is normally obtained. If touched with a piece of human skin, there is copious discharge. The discharge, however, only takes place where there has been mechanical contact. Strong food solutions sometimes seem to cause spontaneous discharge of a few nematocysts. But it is easy to show that this is of no direct significance in the normal reaction. As a direct chemical stimulus, food is far less effective than certain surface-active substances such as bile salts (0.1 per cent), which produce a complete spontaneous discharge of the nematocysts. This action is, however, quite different from the normal response. Cotton wool soaked in dilute bile salts does not adhere to the tentacles ; the nematocysts being discharged, by the diffusing bile salt before contact is made. On the other hand, cotton wool soaked in food solution causes extensive discharge of nematocysts into the mass on contact. This is true even though the food solution employed is far below the concentration for spontaneous discharge. Clean cotton wool, soaked in sea water as a control, produces no discharge.