Molluscs (Mollusca) are a group of invertebrates that includes squid, octopuses, cuttlefish, nudibranchs, snails, slugs, limpets, sea hares, mussels, clams, oysters, scallops, as well as many less well-known animals. Scientists estimate that there are more than 100,000 species of molluscs alive today. This makes them the second largest phylum of animals, having fewer species than only the arthropods.