Mollusca

Mollusca is a massive phylum of invertebrate animals known as molluscs. These creatures are incredibly diverse in their habitats and body structures. They live in oceans, freshwater, and even on land. Molluscs are the second-largest animal phylum on Earth. Only the arthropods have more recognized living species. Scientists currently recognize about 86,600 living species. There are also between 60,000 and 100,000 known fossil species.

To understand how a mollusc works, scientists often look at a "hypothetical ancestral mollusc." This model describes a basic body plan that many species share. The animal is unsegmented and shows bilateral symmetry. A soft body made of muscle contains the internal organs, called the visceral mass. A thick layer of tissue called the mantle covers the upper surface. This mantle secretes a shell made of proteins, chitin, and calcium carbonate.

The mantle also creates a mantle cavity. This is a large space used for breathing and excretion. The anus, genitals, and kidney-like organs called nephridia all open into this cavity. Most molluscs also use a radula to feed. The radula is a rasping tongue covered in tiny teeth. This tool helps them scrape or tear food. In bivalves, like clams, this radula is absent.

Molluscs are divided into several taxonomic classes. The gastropods, which include snails and slugs, are the most diverse. They account for 80% of all classified molluscan species.

Another important group is the cephalopods. This class includes squids, cuttlefish, and octopuses. Cephalopods are among the most neurologically advanced invertebrates. The giant squid is one of the largest known invertebrate species. Bivalves make up about 14% of the total species.

The history of these animals stretches back millions of years. Evidence shows that gastropods, cephalopods, and bivalves appeared during the Cambrian period. This era lasted from 541 to 485.4 million years ago.

Scientists still debate how they emerged from ancestral groups. They also study how they diversified into so many forms. The scientific study of these animals is called malacology.

Molluscs have a significant impact on human life and history. They are a major food source for people worldwide. For centuries, they have provided luxury goods like pearls and sea silk. Some shells were even used as money in preindustrial societies. However, some species can be dangerous. The blue-ringed octopus has a bite that is often fatal.

Other cone snails produce venoms used in neurological research.

There are also serious health and environmental connections. A disease called schistosomiasis is spread by water snails. This condition affects about 200 million people.

Some snails and slugs are also major agricultural pests. If introduced to new environments, they can damage entire ecosystems. While most molluscs live in the sea, non-marine species are very vulnerable. About 42% of all recorded extinctions since 1500 have been molluscs.