Animals in the phylum mollusca share many common traits. These invertabrates have shells whether useful or vestigial. some shells, like one would find with a squid or octopus, have receded deep into the body cavity of the animal. While other animals such as snails still use there shells entirely. Mollusks are the largest marine phylum while they also have expanded to fill niches in freshwater and terrestrial environments. These animals have such a wide diversity of traits that its difficult to find many similar defining characteristics. some of the animals in this phylum can be dangerous and deadly such as a vicious squid or a bit from a poisonous octopus. Others are completely harmless and very unthreatening such as a terrestrial slug.