Cuttlefish, squid, snails and scallops are molluscs, as are clams, sea hares, nudibranchs, mussels, oysters and limpets. As of 2015, there are roughly 100,000 existing mollusc species, which makes them the second-largest phylum of animals, following arthropods.
Mollusks do not have any skeletal systems or backbones, which means they are invertebrates. Molluscs have three main body parts: the foot, the visceral mass and the mantle. The visceral mass is the soft part of the animal. It contains the organism's vital organs, including the heart and digestive system. The visceral mass is protected by the mantle, which is a hard coating of tissue. The foot is usually located on the bottom of the creature. It secretes a slippery mucus, which allows the organism to glide on many surfaces.
It is difficult to determine what defines a mollusc because there are so many of them with varying features. One very common feature is a protective shell that is made from calcium carbonate, chitin and other various proteins. Molluscs are sorted into different categories. There are monoplacophorans, polyplacophorans and placophorans, as well as cephalopods, gastropods, bivalves and scaphopods. Scientists believe that molluscs evolved from an ancient creature that is most similar to the flatworm.