As a general rule the gonophores of Siphonophora do not escape from the parent colony as free-swimming Medusae, but an exception occurs in Velella, which produces a number of small free-swimming Medusae formerly described by Gegenbaur under the generic name Chrysomitra. This Medusa has a velum, a single tentacle, eight to sixteen radial canals, and it bears the gonads on the short manubrium. The Medusa of Velella has, in fact, the essential characters of the Anthomedusae.
Our knowledge of the life-history of the Siphonophora is very incomplete, but there are indications, from scattered observations, that in some genera, at least, it may be very complicated.
The fertilised ovum of Velella gives rise to a planula which sinks to the bottom of the sea, and changes into a remarkable larva known as the Conaria larva. This larva was discovered by Woltereck[[337]] at depths of 600-1000 metres in great numbers. It is very delicate and transparent, but the endoderm is red (the colour so characteristic of animals inhabiting deep water), and it may be regarded as essentially a deep-sea larva. The larva rises to the surface and changes into the form known as the Ratarula larva, which has a simple one-chambered pneumatophore containing a gas, and a rudiment of the sail. In contrast to the Conaria, the Ratarula is blue in colour. With the development of the zooids on the under side of this larva (i.e. the side opposite to the pneumatophore), a definite octoradial symmetry is shown, there being for some time eight dactylozooids and eight definite folds in the wall of the pneumatophore. This octoradial symmetry, however, is soon lost as the number of folds in the pneumatophore and the number of tentacles increase.
It is probable that in the Siphonophora, as in many other Coelenterata, the production of sexual cells by an individual is no sign that its life-history is completed. There may possibly be two or more phases of life in which sexual maturity is reached.
An example of a complicated life-history is found in the Calycophoran species Muggiaea kochii. The embryo gives rise to a form with a single nectocalyx which is like a Monophyes, and this by the budding of a second nectocalyx produces a form that has a remarkable resemblance to a Diphyes, but the primary nectocalyx degenerates and is cast off, while the secondary one assumes the characters of the single Muggiaea nectocalyx. The stolon of the Muggiaea produces a series of cormidia, and as the sexual cells of the cormidia develop, a special nectocalyx is formed at the base of each one of them, and the group of zooids is detached as an independent colony, formerly known as Eudoxia eschscholtzii. In a similar manner the cormidia of Doramasia picta give rise to the sexual free-swimming monogastric forms, known by the name Ersaea picta (Fig. 142). In these cases it seems possible that the production of ripe sexual cells is confined to the Eudoxia and Ersaea stages respectively, but it is probable that in other species the cormidia do not break off from the stolon, or may escape only from the older colonies.
Fig. 142.—Free-swimming Ersaea group of Doramasia picta. B, B, batteries of nematocysts borne by the tentilla; D, dactylozooid; G, gastrozooid; H, hydrophyllium; N, nectocalyx; O, oleocyst; f.t, terminal filament of a battery; t, t, tentilla. The gonozooid is hidden by the gastrozooid. × 10. (After Chun.)
The Siphonophora are essentially free-swimming pelagic organisms. Some of them (Auronectidae) appear to have become adapted to a deep-sea habit, others are usually found in intermediate waters, but the majority occur with the pelagic plankton at or very near the surface of the open sea. Although the order may be said to be cosmopolitan in its distribution, the Siphonophora are only found in great numbers and variety in the sub-tropical and tropical zones. In the temperate and arctic zones they are relatively rare, but Galeolaria biloba and Physophora borealis appear to be true northern forms. The only British species are Muggiaea atlantica and Cupulita sarsii. Velella spirans occasionally drifts from the Atlantic on to our western shores, and sometimes great numbers of the pneumatophores of this species may be found cast up on the beach. Diphyes sp., Physalia sp., and Physophora borealis are also occasionally brought to the British shores by the Gulf Stream.
The Calycophorae are usually perfectly colourless and transparent, with the exception of the oil-globule in the oleocyst, which is yellow or orange in colour. Many of the other Siphonophora, however, are of a transparent, deep indigo blue colour, similar to that of many other components of the plankton.
Most of the Siphonophora, although, strictly speaking, surface animals, are habitually submerged; the large pneumatophores of Velella and Physalia, however, project above the surface, and these animals are therefore frequently drifted by the prevailing wind into large shoals, or blown ashore. At Mentone, on the Mediterranean, Velella is sometimes drifted into the harbour in countless numbers. Agassiz mentions the lines of deep blue Velellas drifted ashore on the coast of Florida; and a small species of blue Physalia may often be seen in long lines on the shore of some of the islands of the Malay Archipelago.