Short answers, pulled from the story.
Mollusca is a phylum of soft-bodied protostome invertebrate animals known as molluscs or mollusks. There are 86,600 recognized living species, making it the second-largest animal phylum after the arthropods. An estimated 60,000 to 100,000 additional fossil species are known.
The four most universal features of modern molluscs are a soft body composed almost entirely of muscle, a mantle with a significant cavity used for breathing and excretion, a rasping tongue called a radula that is absent only in bivalves, and a distinctive nervous system structure. The hypothetical ancestral mollusc used in textbooks has a single limpet-like shell on top and a single muscular foot underneath.
Molluscs are typically divided into seven or eight classes, two of which are entirely extinct. The living classes are Gastropoda with about 70,000 species, Bivalvia with about 20,000, Polyplacophora with about 1,000, Cephalopoda with about 900, Scaphopoda with about 500, Aplacophora with about 320, and Monoplacophora with 31. Gastropods account for about 80% of all classified molluscan species.
The radula is a muscular rasping tongue bearing rows of chitinous teeth that are replaced from the rear as they wear out, and it is unique to molluscs with no equivalent in any other animal. It primarily scrapes bacteria and algae off rocks and works with a cartilaginous organ called the odontophore.
A few molluscs are dangerous. Blue-ringed octopuses of the genus Hapalochlaena bite only when severely provoked, but their venom kills 25% of human victims, and all cone snails are venomous. Schistosomiasis, a disease carried by freshwater snail hosts, infects an estimated 200 million people in 74 countries.
Molluscs have supplied pearls, mother of pearl, Tyrian purple dye, and sea silk for centuries. Tyrian purple, made from the ink glands of murex shells, fetched its weight in silver in the fourth century BC according to Theopompus, and shells such as cowries were used as money in some preindustrial societies.
Good evidence places gastropods, cephalopods, and bivalves in the Cambrian period, 541 to 485.4 million years ago. Helcionellids with snail-like shells first appear in Early Cambrian rocks from Siberia and China, meaning shelled molluscs predate the earliest trilobites, while the Late Cambrian Plectronoceras is the earliest undisputed cephalopod fossil.