Microanatomy of the subumbrellar motor innervation in Aglantha digitale (Hydromedusae: Trachylina)

The hydrozoan medusa Aglantha digitale (Müller 1776) has eight syncytical giant motor axons, up to 40 micrometers in diameter, running from the margin, up the inside of the bell towards the apex. Giant motor axons injected with Lucifer Yellow CH are connected with lateral neurons running circumferentially across the subumbrellar muscle. These processes fill with the dye. Bundles of 20 to 50 small dye-coupled neurons extend circumferentially along the margin for up to 0.85 mm. Giant motor axons injected with horseradish peroxidase divided into a few short branches on entering the inner nerve ring. Here the giant motor axon forms both chemical synapses and gap junctions with neurons that also send their axons into the inner nerve ring. In this region the inner and outer nerve ring are connected by axons passing through openings in the intervening mesoglea.