The rhopalia are lodged in the notches be tween the marginal lobes of the umbrella, and each rhopalium is covered over by a little protecting flap or lappet.
Highly developed eyes, with ectodermal pigment and lens, are found also on the rhopalia of Paraphyllina (Maas [81).
- Medusae more or less cubical in form, with four perradial rhopalia alternating with four interradial tentacles or groups of tentacles; oral arms short; stomach a wide cavity bearing four interradial groups of phacellae and giving off four broad perradial pouches completely separated from each other by four interradial septa (i.e.
- Medusae of deep pyramidal form, often sessile, attached by a stalk developed from the centre of the exumbral surface; rhopalia absent or represented by colletocystophores.
- Free medusae with rhopalia of the normal type; the exumbrella is divided by a circular, so-called coronal groove, into two parts, a central portion, which is conical, thimble-shaped, or domed in form, and a peripheral portion, the pedal zone, which bears the marginal lobes, tentacles and rhopalia; the pedal zone is subdivided into areas termed pedalia, from each of which arises a tentacle or rhopaliurn in the interspace between two adjacent lobes of the margin.