The Diphyidae are a family of siphonophores. 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.