Hydromedusae have a distinct tetramerous radial symmetry with most structures occurring in fours or multiples of four. The arched saucerlike body is referred to as the bell . It has a convex upper, aboral surface , and a concave lower, oral surface . The aboral surface is smooth and rounded. In Obelia , the velum, which is characteristic of most hydromedusae, is rudimentary and will not be apparent.
Numerous tentacles extend from the margin of the bell.
It may be necessary for you to look at several medusae to get a good side view of the manubrium. The mouth is in the center of its free, oral end and is flanked by four short oral arms . The manubrium and oral arms are well supplied with cnidocytes. The mouth opens into the a gastrovascular space within the manubrium which in turn empties into the stomach . The stomach is in the center of the bell and usually stains darker than the surrounding tissue.