Siphonophore

It lives in the Arctic and other cold, deep waters, swimming independently in mid-ocean.Siphonophores Retrieved 2011-10-01.

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.Grzimek B. Schlager N. & Olendorf D. 2003. Grzimek's Animal life encyclopaedia. Thomson Gale. 

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.

The zooids fit together so closely that the colony looks like a single individual. It was a triumph of 19th century biology to discover the real nature of the siphonophores.

Bathyphysa conifera
Portuguese man o' war

 References 

Category:Cnidarians