Distribution of Nerve Elements Showing FMRFamide‐like Immunoreactivity in Hydromedusae

Immunofluorescence tests with antisera to various peptides reveal the presence of a FMRFamide‐like peptide in neurons of hydromedusae belonging to three orders. An avian pancreatic polypeptide‐like peptide may also be present in certain neurons. Distribution of FMRFamide‐like immunoreactivity (Fa‐IR) has been studied in Proboscidactyla (O. Limnomedusae), Phialidium and Aequorea (O. Leptomedusae) and Aglantha (O. Trachymedusae). These findings are compared with results obtained with Polyorchis (O. Anthomedusae) by Grimmelikhuijzen and Spencer [ J. comp. Neurol. 230 , 361 (1984)]. All species show Fa‐IR neurons in the tentacles linked by interconnecting neurites running in the outer marginal nerve ring. A Fa‐IR plexus is present in the manubrium and, except in Aglantha , this system merges with a plexus associated with the subumbrellar radial muscles. As in Polyorchis , the swimming motor neurons are unstained. In contrast to this species, only a small percentage of the neurites composing the nerve rings are stained. The giant axons of Aglantha show no Fa‐IR. The cnidothylaces of Proboscidactyla contain neurons reacting with antisera to FMRFamide and APP. Present evidence suggests that in hydromedusae Fa‐IR is confined to distinct subsets of neurons. These appear to be either sensory units or units supplying smooth muscles, but they are not involved in the innervation of striated muscles.