Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks[a] (/ˈmɒləsk/). Around 85,000 extantspecies of molluscs are recognized.[3] The number of fossil species is estimated between 60,000 and 100,000 additional species.[4] The proportion of undescribed species is very high. Many taxa remain poorly studied.[5]
The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small.
The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords,or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.
Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.
Molluscs have been and still are an important food source for anatomically modern humans. A risk of food poisoning exists from toxins that can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies.
Cornu aspersum (formerly Helix aspersa) – a common land snail
Fossilized ammonite displayed at the National Museum of the Philippines
Diversity and variability of shells of molluscs on display
About 80% of all known mollusc species are gastropods (snails and slugs), including this cowry (a sea snail).[17]
Anatomical diagram of a hypothetical ancestral mollusc
Play media
A 50-second video of snails (most likely Natica chemnitzi and Cerithium stercusmuscaram) feeding on the sea floor in the Gulf of California, Puerto Peñasco, Mexico
Snail radula at work = Food = Radula = Muscles = Odontophore "belt"
Simplified diagram of the mollusc nervous system
The use of love darts by the land snail Monachoides vicinus is a form of sexual selection
The tiny Helcionellid fossil Yochelcionella is thought to be an early mollusc[45]
Spirally coiled shells appear in many gastropods.[15]: 300–343