<p>The Cephalopoda is an <em><strong>ancient</strong></em> and very successful group of the Mollusca. Cephalopods have been among the dominant large predators in the ocean at various times in geological history. Two groups of cephalopods exist today: The Nautiloidea with a few species of the pearly nautilus, and the Coleoidea, containing the squids, cuttlefishes, octopods and vampire squids, which is represented by about 700 species. Cephalopods are the most active of the molluscs and some squids rival fishes in their swimming speed. Although there are relatively few species of living cephalopods, they occupy a great variety of habitats in all of the world's oceans. Individual species are often very abundant and provide major targets for marine fisheries. </p> <p> Cephalopods first appeared about 500 million years ago in the Upper Cambrian Period. Although considerable uncertainity still exists, the two extant lineages may have separated 470 mya with the possible origin of the Bactritida or earlier. The long separation of the two lineages has, today, resulted in lineages with cephalopods that are very different in structure.</p>