Medusal stage or chrysomitra.
The medusa [V.velella-meduse ] is released from the gonozooid before it is fully developed and initially measures about 1.5 mm in height. The mouth of the manubrium remains closed for several days. The manubrium is conical and has a quadrate base, the mouth is tubular. There are four radial canals. Later, two pairs of opposite perradial tentacles appear, one short adaxial and one long abaxial; each tentacle bears a large terminal cnidocyst cluster. The other two perradial marginal bulbs remain without tentacles. The gonads are irregularly arranged perradially and interradially.
Supposedly, the medusa sinks down through the water column to depths of 600-1000 m. Eventually the gonads become mature and the sexual products are released; the chrysomitra then dies. Though little is known of the early stages, they are presumed to develop at depth. The first identified larva is the conaria, with a pair of short aboral tentacles, a rudimentary float and a conspicuous crimson cone which secrets oil droplets. The oil makes the larva buoyant and it ascends slowly to the sea surface. Meanwhile, the conaria transforms into a rataria larva and arrived at sea surface, the hollow adult tentacles appear and the float develops and the sail appears. The hydranth then gradually increases in size.
Hydroid stage or hydranth.
The leptolidV. velella is known by the floating hydranth [V.velella-hyd], which is, in fact an up side down floating polyp colony. As such, it is a pleustonic animal provided with an elliptical or nearly quadrangular shaped float that has a cartilaginous consistency. The float bears a diagonally placed triangular fin on its upper surface that acts as a sail.
The hydranth of V. velella occurs in two dimorphic forms, depending on the position of the sail on the float, resulting in sailing either to the left or to the right of the wind direction [V.velella-sail]. Beneath the float are the hydranths, differentiated into three forms [V.velella-section]:  the central siphon for food digestion, surrounded by numerous  small blastostyles and many short,  submarginal tentacles bearing numerous nematocysts and functioning for food capturing. The blastostyles may ingest food, but they also bear the gonophores producing the medusae.
The cavities of the various siphons and tentacles are connected into a widespread gastrovascular system, that forms a network mainly concentrated above the central siphon; this area could be considered to have an excretory function and is thus called a “liver”.
Medusa: the smallest size is ca. 1.5 mm.
Hydranth: up to 10 cm in length; the “sail” is up to 40 mm long and 20 mm high.
The meduse occurs from surface to ca. 1000 m depth, related to development and life history. The hydranth is floating on the sea surface.
Distribution in the North Sea
V. velella is a warm water species and therefore unlikely to occur regularly in the North Sea. The meduse is not reported from the area; because of just a few records, the hydranth must be considered to be an exception in the North Sea.
In the summer of 2002, complete or cartilagous remnants of Jack-sail-by-the-wind (= the english vernacular name of the hydranth) have been found washed ashore on the Belgium coast and the coasts of Walcheren and Texel, The Netherlands (Cadée and Cadée-Coenen, 2002). The chronological order of the findings on a more or less S-N line — Belgium first, Texel last — suggests that the hydranths originated from the Channel and have entered the southern North Sea through the Dover Strait/Pas de Calais and were then transported northward by surface currents.
The Jack-sail-by-the-wind is regularly reported from mass strandings on the Irish and British south coasts.
Global species of warmer waters.
The full life cycle of Velella is not known, as the earliest developmental stages take place at great depth. It is not known whether there is a planula larva in the cycle.
[After Kirkpatrick and Pugh, 1984 — except distribution in the NS]