The Class Scaphopoda contains about 400 species of molluscs called tooth or tusk shells, all of which are marine. Scaphopods range in size from 4 to 25 cm long (although most species are 2.5-5 cm in length). In comparison to other molluscs, scaphopods are a "young" group, appearing in the fossil record about 450 million years ago during the Ordovician period, and present evidence suggests that they had the same ancestor as the bivalves.
Tooth and tusk shells live sedentary lives buried in sand or mud substrates, mostly in water as deep as 6 km. Their shells grow linearly as a hollow, curved tube with an opening at each end; water enters and leaves the narrower end, which protrudes above the substrate. Unlike many molluscs, scaphopods lack ctenidia (gills), a heart and a circulatory system, and blood circulates through the various sinuses of the hemocoel as a consequence of the foot's rhythmic movements. Small food particles (such as foraminiferans) from the surrounding sediment are captured by 100-200 sticky tentacles called captacula, which are then transported to the mouth.