[Hydromedusae] In Narcomedusae the tentacles leave the umbrella at some distance along the margin just above the clefts separating the marginal lappets. At the edges of the clefts the subumbrellar and exumbrellar ectoderm fuse without interposition of mesoglea, forming the peronial grooves, which are invaded by tentacular ectoderm, together making up ectodermal strands rich in cnidocysts, muscles, and nerves: the peronia. At the base of the peronia the margin of the umbrella lappets remain curved, giving the umbrella its lobed appearance.