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The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar. [16] Mollusks live in most terrestrial, freshwater, and marine habitats. However, the majority of species live in the ocean. They can be found in both shallow and deep water and from tropical to polar latitudes. Mollusks are a major food source for other organisms, including humans. You may have eaten mollusks such as clams, oysters, scallops, or mussels.
Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood. a b Jacobs, D.K.; Wray, C. G.; Wedeen, C. J.; Kostriken, R.; Desalle, R.; Staton, J. L.; Gates, R.D.; Lindberg, D.R. (2000). "Molluscan engrailed expression, serial organization, and shell evolution". Evolution & Development. 2 (6): 340–347. doi: 10.1046/j.1525-142x.2000.00077.x. PMID 11256378. S2CID 25274057.Chisholm, Hugh, ed. (1911). "Molluscoida". Encyclopædia Britannica. Vol.18 (11thed.). Cambridge University Press. p.675.
Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period, 541 to 485.4 million years ago (mya). Before that, the evolutionary history of molluscs' emergence from the ancestral Lophotrochozoa is still unclear. It has become clear that the fossil taxon Helcionelloida does not belong to the class Gastropoda. It is now a separate class in the Mollusca. Parkhaev (2006, 2007) created the class Helcionelloida, whose members were previously treated as "Paleozoic mollusks of uncertain systematic position" by Bouchet & Rocroi. [5] [6] Uses [ change | change source ]
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