Like all siphonophores, the Portuguese man o' war is colonial: each man o' war is composed of many smaller units (zooids) that hang in clusters from under a large, gas-filled structure called the pneumatophore. New zooids are added by budding as the colony grows. As many as seven different kinds of zooids have been described in the man o' war: three of the medusoid type (gonophores, nectophores, and vestigial nectophores) and four of the polypoid type (free gastrozooids, tentacle-bearing zooids, gonozooids and gonopalpons). However, naming and categorization of zooids varies between authors, and much of the embryonic and evolutionary relationships of zooids remain unclear.