These are colonial siphonophores with two nectophores (swimming bells) arranged one behind the other.

The front one includes a somatocyst (extension of the gastrovascular system), while the hind one does not.

The somatocyst often contains an oil droplet for buoyancy control.

A nectosac (central cavity with muscular walls) in each nectophore allows the organism to swim efficiently.

[1] The World Register of Marine Species includes the following taxa in the family Diphyidae:[2]