Second-largest animal phylum after the arthropods
Mollusca holds about 86,600 living species—second only to arthropods—and maybe 60,000 to 100,000 more as fossils. Roughly 23% of named marine organisms are molluscs. Soft muscular bodies, a mantle cavity, usually a radula, and a shared nervous plan unite snails, clams, and cephalopods.
Diversity spans size, anatomy, behaviour, and habitat: seas dominate, yet freshwater and land lineages thrive. Traditional classes include gastropods, bivalves, and cephalopods among others. Coelomates they are, but the body cavity stays small compared with the spacious mantle chamber used for breathing and excretion.
Bivalves ditch the radula; everyone else scrapes with that chitinous ribbon. Muscle makes almost the whole soft body, which is why a squid can be mostly jet and a snail mostly foot. The same phylum blueprint yields filter-feeding oysters and hunting octopuses.
Inside, blood washes through an open cavity called the hemocoel rather than a closed network of vessels. Food is rasped up by the radula and moved along a gut where mucus and tiny muscle-driven cilia do much of the work. Most molluscs have eyes, and all can sense chemicals, vibration and touch. Nearly all lay eggs, which may hatch as trochophore larvae, as more elaborate veliger larvae or as miniature adults, and the simplest species rely on fertilization outside the body.
Gastropods, cephalopods and bivalves all appear in the Cambrian, between 541 and 485.4 million years ago. People have long eaten molluscs, though toxins that build up in some shellfish can cause food poisoning, and pearls count among their luxury products. A few are dangerous: the blue-ringed octopus's bite is often fatal, and some large tropical cone shells can kill with their sting. The name traces through French to Latin mollis, soft, first applied by J. Jonston in 1650 to a group of cephalopods.
Source: Mollusca
