15),the polyps produce buds right and left alternately, so that the hydranths are arranged in a zigzag fashion, forming a " scorpioid cyme," as in Obelia and Sertularia.
An important variation is seen, in the form of the hydrotheca itself, which may come off from the main stem by a stalk, as in Obelia, or may be sessile, without a stalk, as in Sertularia.
Two of the commonest British hydroids belong to this family, Obelia and Clytia.
Obelia forms numerous polyserial stems of the characteristic zigzag pattern growing up from a creeping basal stolon, and buds the medusa of the same name.
The tentacles are usually hollow, rarely solid (Obelia) .