The word mollusc comes from a Latin word meaning "soft", which refers to the soft coat that covers the molluscs, which then secretes the enzymes to form the calcium carbonate shell that protects their young . The phylum Mollusca consists of invertebrates containing a kind of calcium carbonate shell to cover their soft coat which secretes the enzymes that make up this shell. Molluscs are one of the most diverse phyla in the world, second only to insects. The earliest species of molluscs dating back to the Cambrian period (about 542 million years ago) include chitons and conchifers. Most of the fossils discovered are extremely small, measuring only about 1 to 5 mm. The oldest known cephalopods, which include squid, octopus and cuttlefish, date back to much later times, at the end of the Cambrian, which ended around 488 million years ago. The fossil record provides very little evidence to prove how mollusks arose and evolved into what they are today. Due to the paucity of evolutionary evidence, scientists have turned to comparative molecular biology. Biologists have deduced from studies that Phylum Mollusca and Phylum Annelida, made up of segmented worms, share a common ancestor. The strongest evidence that they share a common ancestor is found in the embryonic stage. They both have a pear-shaped larva called a trochophore in the first stage of development. In both phyla, these larvae have cilia that protrude from each end of the trochophore and encircle the middle. These cilia work by propelling the trochophore through the water and attracting food into their mouth. Free-swimming trochospheres also aid in the dispersal of other marine molluscs. However, terrestrial molluscs and many marine annelids develop...... middle of paper ......the ability of a mollusc's muscular foot to grasp objects, such as food, has been adapted to this which today is the tentacles of an octopus or a squid. The calcium carbonate shell found in most molluscs comes in varying sizes, shapes, and colors, but most have one function: to protect the mollusk's soft mantle and prevent predators from eat them. Works Cited Campbell, Dana and Jennifer Hamac. “Molluscs – Details.” Encyclopedia of life. Encyclopedia of Life, nd Web. March 13, 2014. Foster, Bethney. “Life cycle of molluscs”. EComment. Demand Media, August 26, 2009. Web. March 25, 2014. Salvini-Plawen, Luitfried Von. “Mollusk (animal phylum).” Encyclopedia Britannica Online. Encyclopedia Britannica, nd Web. March 25, 2014. “Phylum Mollusca.” Phylum Molluscs. Np, and Web. March 25, 2014. “Mollusk”. - Encyclopedia of the New World. Np, April 30, 2006. Web. March 29. 2014.