Nervous control of respiration: oxygen-sensitive elements in the prosoma of Limulus polyphemus.

1. Responses of oxygen-sensitive units in the prosomal haemal nerve of Limulus polyphemus were examined while varying the oxygen content of sea water bathing the intercoxal cuticle. 2. When exposed to high oxygen levels these units units maintained a continuous background discharge of spikes. Unit activity was inhibited when oxygen content decreased. Upon reintroduction of oxygen tonic spike discharge resumed. 3. Mechanosensitive units with receptive fields on the prosomal shield or intercoxal cuticle were also present in the haemal nerve. Neither the mechanosensitivity nor the background discharge of these units was affected by changes in oxygen content. 4. It is proposed that the oxygen-sensitive respiratory reflexes of Limulus are an adaption to existence in the tertidal zone. Published observations of the respiratory stress responses of many intertidal animals support this hypothesis.