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.
In Charybdaea there are no less than six ocelli on each of the four rhopalia (fig.
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.
- With sixteen marginal lobes, four rhopalia and twelve tentacles; the rhopalia are interradial.
With sixteen marginal lobes, four rhopalia and twelve tentacles; the rhopalia are perradial in position, corresponding to the angles of the stomach.
With twelve marginal lobes, six rhopalia and six tentacles.
- With eight marginal lobes, four rhopalia and four tentacles.
- With sixteen or thirty-two rhopalia, marginal lobes and tentacles often very numerous.
With sixteen marginal lobes, eight rhopalia and eight tentacles.