There are ten classes of mollusks, of these eight are still living. This diverse group of animals includes varieties from deep sea worm-like creatures, to clams and snails, to the colorful and strange cephalopods. Mollusks first appeared in the Middle Cambrian period over 500 MYA in the Burgess Shale. Living cephalopods like the octopus, cuttlefish and squid have a shell that has been internalized, while the nautilus shell is external. Ammonoid cephalopods first appeared in the fossil record about 400 MYA, and became extinct at the end of the Cretaceous. Ammonoid cephalopods had a hard external shell like the nautilus, however they were more closely related to the squid and octopus. Today these mollusks, both extinct and extant, are admired and valued for their beautiful, varied and colorful shells.