[70], Monoplacophorans("limpet-like", "living fossils"), Gastropods(snails, slugs, limpets, sea hares), Cephalopods(nautiloids, ammonites, octopus, squid, etc. [75], The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticle. [15] The whole soft body of bivalves lies within an enlarged mantle cavity. Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable. A possible "family tree" of molluscs (2007). [12], The mantle cavity, a fold in the mantle, encloses a significant amount of space. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. This phylum is dispersed widely throughout the habitats provided to it by our planet. The proportion of undescribed species is very high. The sorting process is by no means perfect. Nudibranch Sea Slugs: Species, Behavior, and Classifications, Cephalopod Class: Species, Habitats, and Diets, Animals of Australia's Great Barrier Reef. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. [12], 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. [92] When used for commercial transactions, they functioned as commodity money, as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or "goldrush" behavior appeared.[93]. Just imagine how easily the average house cat, used to picking off skittering mice, can devastate a near-motionless colony of snails. [45] However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated. [15] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. Other characteristics that commonly appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species. The brain, in species that have one, encircles the esophagus. [29] Most stay within the sediments at the bottom of water bodies, although a few—such as cephalopods—are free swimming. The two methods of culturing pearls insert either "seeds" or beads into oysters. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla. [53], However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China,[54][55] are thought to be early molluscs with rather snail-like shells. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly [12], In some forms the shell contains openings. Any group that embraces squids, clams, and slugs present a challenge when it comes to formulating a general description. Gastropods and bivalves may be the most common mollusks, but cephalopods (the family that includes octopuses, squids, and cuttlefish) are by far the most advanced. The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of the living molluscs. 31 (3–4): 144..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .id-lock-free a,.mw-parser-output .citation .cs1-lock-free a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited a,.mw-parser-output .id-lock-registration a,.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription a,.mw-parser-output .citation .cs1-lock-subscription a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}.