The unassuming phylum of soft-bodied animals known as molluscs are actually quite numerous (between 85,000-200,000 known species), and are second only in numbers to arthropods. Though many classes of mollusk are documented, only three extant extant varieties are well-known to the average human: the Gastropods (snails and slugs), the Bivalves (oysters, clams, mussels, and kin), and the Cephalopods (squids, octopuses, nautiloids and kin, including the fossiliferous, extinct Ammonites). Molluscs have great morphological diversity, from the beaked and multi-armed, predatory squids, to the slow and shelled, herbivorous garden snails. All molluscs share three unique characteristics: the organization of their nervous systems, a mantle used for breathing and excretion, and a radula (a tongue-like structure used mostly by herbivorous molluscs to scrape algae and bacteria). Most molluscs possess a calcium-carbonate based shell (lost in later groups like squids and octopuses) that is used for protection, and is thought to have originated with the first ancestral mollusc. The oldest known mollusk fossils date back to the Cambrian period, approximately 541-485 million years ago. Today’s molluscs still reside mostly in marine environments, with some having adapted to freshwater and terrestrial habitats.
Cephalopods: Ammonites, Squids & kin