Discussion:Molluscs 2BIbDF09 10/11
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Intro Molluscs are invertebrates animals that live in water or in moist environmnent, and mostly in marine environment. There is an estimated number of 100'000 species in the phylum Mollusca. Molluscs appeared in the earliest Cambrian period 545 million years ago. They have a bilateral symmetry and are soft-bodied animals, but most are protected by a hard shell. Some molluscs have external hard shells, like Nautiluses, Snails or Clams, but other have reduced or internal shells, like Cuttlefish or Squids. The rest just lost it during evolution, like Octopuses or Slugs. A shell is not an exoskeleton; it is in fact a protection against predators or the dangers of their environment. If it was an exoskeleton, molluscs couldn't live without it but they can, so they have either a hard shell or an internal skeleton. We can mainly define four different groups of molluscs (cf. appendix molluscs 2); the Gastropods (cf. appendix molluscs 3), the Bivalves (cf. appendix molluscs 4), the Polyplacophores (cf. appendix molluscs 5) and the Cephalopods(cf. appendix molluscs 6).All of them have the same body plan separated in three parts. The first part is the foot which is the strong, muscular portion used for locomotion. The second one is the visceral mass, a soft-bodied portion that contains internal organs and the third one is the mantle, that covers and protect the visceral mass and, for those who have shells, secrets the shell. (cf. appendix molluscs 1)