Mollusks (British spelling: molluscs) are a large and diverse phyla of invertebrate animals, featuring over 110,000 species. Their phylum name, Mollusca, means "thin-shelled," though many species lack shells entirely. Mollusks include clams, oysters, scallops, mussels, snails, squid, octopuses, slugs, nudibranchs, sea hares, and several classes of deep-sea wormlike creatures. Some mollusks — snails and slugs — have even adapted to life on land. Mollusca is one of the "Big Nine" animal phyla, alongside Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata.
For thousands of years, mollusks have been one of mankind's favorite sea creatures alongside fish and crustaceans, providing us with their meat and decorative shells. Shell middens, huge stacks of shells left by prehistoric peoples, line many the coastal zones of the world, providing important evidence about our ancestors' migration patterns and lifestyles. Some middens are tens of thousands of years old.
Mollusks include a variety of record-holders and unusual animals. There's the Colossal Squid (Mesonychoteuthis hamiltoni), the largest of all modern invertebrates, which is at least 14 m (46) feet in length, the Vampire Squid (Vampyroteuthis infernalis, lit. "vampire squid from hell"), a phylogenetic relict with characteristics of both squids and octopuses, and nudibranchs (suborder Nudibranchia), some of the most beautiful and colorful marine animals that exist. Mollusks range in size from less than 1 mm, in the case of some micromollusks, to the Colossal Squid, which is capable of killing sperm whales for prey.
The defining characteristics of mollusks are a muscular foot, especially obvious in the case of snails, and the mantle, a protective dorsal body wall covering the main body from the outside. Between the mantle and the main body is a mantle cavity, containing the mollusk's gills, anus, and other organs. The mantle cavity may be used as a feeding structure (bivalves), respiratory chamber (all mollusks), brood chamber (numerous), or locomotory organ (squids and some clams).