On triggering and control of cnidocyst discharge

Ultrastructural and in vivo observations in several planktonic coelenterates show that the cnidocystes, always associated with a receptor pole (the cnidocil itself or an accessory sensory cell) may have no link with the nervous system, or may be associated with it.

On the basis of these observations we propose that the cnidocytes of physonect siphonophores can be placed in two categories of receptor‐effectors both sensitive to stimuli received by the cnidocil. The cnidocytes of the first category, with defensive functions, localized on the pneumatophore, nectophores and bracts, have no link with the nervous system and are typical independent receptor‐effectors. In contrast, those of the second category whose distribution on the fishing tentacles and gastrozolds, implies predatory functions, are connected with the nervous system which may modulate or control their thresholds of excitability. Thus, in one and the same animal, depending on their function, the cnidocytes may or may not be linked with the nervous system, a difficult and long debated question in most of the cnidarians.