Chondrophore
The chondrophores or porpitids are a small and unusual group of hydrozoans. They are the family Porpitidae.
They all live at the surface of the open ocean, and are colonies of carnivorous, free-floating hydroids. Their life-style in the plankton is similar to that of pelagic jellyfish.
The chondrophores look like a single organism but are cooperatives of polyps.
The most familiar members of the family Porpitidae are the blue button (Porpita porpita) and the by-the-wind sailor (Velella velella).
Structure
Chondrophores may look like a jellyfish, but they really are not. Like the siphonophores they are not a single animal. They are a colony of clones, genetically identical zooids. These zooids are small, highly modified individual polyps. Though structurally similar to other cnidarians, the zooids do not live by themselves: they are attached to each other. Each type of zooid depends for survival on the others doing what it cannot do by itself.
This kind of set-up is also found in the siphonophores: the Portuguese man o' war. They evolved independently, and are classified in different orders.
Fossil record
A rare soft-bodied fossil was got from Mississippian strata in northeastern Kentucky. It was interpreted as a chondrophorine float. The origin of the group was probably in the Neoproterozoic era, some 650540 million years ago.
References
Cnidarians