The sub-umbrella invariably shows a velum as an inwardly projecting
ridge or rim at its margin, within the circle of tentacles; hence the
medusae of this sub-class are termed craspedote. The manubrium is
absent altogether in the fresh-water medusa _Limnocnida_, in which the
diameter of the mouth exceeds half that of the umbrella; on the other
hand, the manubrium may attain a great length, owing to the centre of
the sub-umbrella with the stomach being drawn into it, as it were, to
form a long proboscis, as in _Geryonia_. The mouth may be a simple,
circular pore at the extremity of the manubrium, or by folding of the
edges it may become square or shaped like a Maltese cross, with four
corners and four lips. The corners of the mouth may then be drawn out
into lobes or lappets, which may have a branched or fringed outline
(fig. 27), and in _Margelidae_ the subdivisions of the fringe simulate
tentacles (fig. 24).
[Illustration: After O. Maas, _Medusae_, in Prince of Monaco's series.