Gastropod

Gastropods are a vast and incredibly diverse group of invertebrates.

They belong to the phylum Mollusca. Most people know them as snails or slugs. They are the second-largest animal class on Earth. Only insects have more species.

Their history is very long. The fossil record shows they existed during the Late Cambrian period. This makes them one of evolution's most successful lineages. They have adapted to nearly every environment on our planet.

A key feature of gastropods is a process called torsion. This happens during their early development. The visceral mass, which contains the internal organs, rotates 180 degrees to one side. This rotation is not related to shell coiling. Instead, torsion changes the animal's internal symmetry. The anus moves from the rear to a position above the head. This makes the animal asymmetrical. Most growth occurs on the left side. This process likely serves a defensive purpose. It allows the animal to retract its head into its shell more easily.

Gastropods show many different body types. Many species have a single, coiled shell. This shell often opens on the right side, which is called dextral coiling.

However, some families are sinistral. This means their shells and internal anatomy are left-coiled. Some gastropods, like limpets, have simple conical shells. Others, called slugs, have no shell or a very small internal one. These slugs have streamlined bodies. Some marine slugs, called nudibranchs, have feathery gills on their backs. They may also breathe through their skin.

Scientists have used different names for these animals over time. In 1795, they were described as "gasteropodes" in scientific literature. This name comes from Greek words for "stomach" and "foot." It describes how the foot is positioned below the guts. They were also called univalves. This term means "one valve." It helped distinguish them from bivalves, like clams, which have two shells. Today, we recognize 721 different families of gastropods.

This includes 476 living families and 245 extinct families.

The diversity of gastropods is staggering. Estimates of living species vary widely. Some sources suggest there are 65,000 to 80,000 living species. Other estimates go as high as 120,000. There are about 24,000 described species of land gastropods. Marine gastropods number about 30,000 species. Freshwater and brackish species number about 5,000.

Even in the deep sea, they thrive. Some live in trenches 10,000 meters below the surface. Because so little of the deep ocean has been studied, many species are still waiting to be discovered.

Gastropods use unique tools to survive and eat. Most have a radula. This is a structure used to rasp or scrape food. For example, limpets use a hard radula to eat seaweed from rocks. Many marine species also have a siphon. This is a tube that extends from the mantle edge. They use the siphon to draw water into their mantle cavity. This allows them to "taste" the water to find prey.

This is especially helpful for predators and scavengers. They also have sensory organs like eyes and tentacles. In land snails, the tips of the tentacles have olfactory organs for smelling. Some have complex eyes, while others only see light and dark.

These animals are connected to many different biological systems. They inhabit marine, freshwater, and terrestrial ecosystems. You can find them in deserts, mountains, and even hydrothermal vents. Some gastropods are even parasites. They can be moved between habitats by other animals, such as birds.

Their ability to live in such different places shows how adaptable they are. Whether they are building shells from calcium carbonate or using protein to make thin shells, they continue to succeed. They remain a vital part of the Earth's biological diversity.