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. [Illustration: 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 {304}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.