Cephalopoda is a class of exclusively marine, present-day mollusks that evolved during the Cambrian Period from a benthonic ancestor, probably a herbivore monoplacophoran mollusk. After 500 million years of evolution the result is a group of organisms with a variety of life-styles (there are benthonic forms, such as octopuses; nectobenthonics, such as cuttlefishes; and nectonics, such as squids), high and fast mobility (performed by a characteristic jet propulsion mechanism), a highly developed nervous system, a relatively short life-span, and a very fast growth for invertebrate standards, all supported by their carnivorous habits. Their biomass, estimated in more than 49 billion long tons (500 million metric tons) per year, together with their voracity, makes them an important component of the marine food webs. Moreover, different cephalopod species constitute an important fishery resource, representing more than 2% of world marine captures.