Siphonophore

The Siphonophores are an order of cnidarian invertebrates in the class Hydrozoa. The Portuguese man o' war is an example. 
A siphonophore is not a single animal. It is a colony of four kinds of zooids. Zooids are very small, highly modified individuals. All the zooids in a colony are genetically identical. These zooids are specialized polyps and medusoids. 
Though structurally similar to other cnidarians, the zooids do not live by themselves: they are attached to each other. Each type of zooid is not self-sufficient. It depends for survival on the others doing what it cannot do by itself.
So close do the zooids fit together that the colony looks like a single individual. It was a triumph of 19th century biology to discover the real nature of the siphonophores.