Class Gastropoda are the largest group of mollusks, with between 40,000 and 75,000 species. These are also the only mollusks that live on land. Gastropods, like class Schapoda, lack gills, and use the mantle as a makeshift lung for extracting oxygen and breathing. Most species have coiled shells, and this class has evolved tentacles and eyes. This class includes marine animals like the sea slug and terrestrial animals like the slug and snail.

Gastropods are in the phylum Mollusca. All mollusks have a muscular foot used for moving and a mantle, an outgrowth that covers the animal. Many mollusks have an external calcium carbonate shell that is produced by the mantle. The gill of a mollusk extracts oxygen from the water and disposes of waste. All species of the phylum Mollusca have a complete digestive tract, spanning from the mouth to the anus. Many also have a radula, a unique organ composed of chitin, in the mouth. The radula allows the animal to scrape food from surfaces, especially the ocean floor, by sliding back and forth. Mollusks have a coelom, but the coelom is made from cell masses, making all species in this phylum protosomes. All organs are suspended in this coelom, between the outer covering and the digestive tube of the animal. However, animals in this phylum are unique to coelomates in that they lack body segmentation. There are seven classes in this phylum.

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