Abstract
ABSTRACT Spontaneous electrical potentials from Obelia geniculata hydranths were recorded using fine-tipped suction electrodes. The primary behavioural responses of the hydranths were observed and correlated with specific electrical potentials: the contraction potential (KP) associated with hydranth withdrawal, the mouth-opening potential (MOP) associated with mouth opening, and the tentacle contraction potential (TKP) associated with oral flexion of individual tentacles. KPs occurred in rhythmic bursts while the TKP and MOP responses were infrequent and non-rhythmic. Two other species, O. longissima and Gonothyraea loveni, were shown to produce the same three types of potentials with corresponding behavioural responses. .The MOP activity may occasionally drive the KP responses within a hydranth, but the coupling is very loose. None of the other possible interactions within a hydranth was observed. .No interaction of any of the potentials between adjacent hydranths was found in the absence of external stimuli. .It seems likely that the KPs in Obelia are homologous with the contraction potential (CP) in Hydra and the hydranth potential (HP) in Tubularia. MOP and TKP activity do not show apparent homologies with potentials of the other hydroids Hydra, Tubularia and Cordylophora.
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