Caecum (gastropod)

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Caecum is a genus of very small sea snails in the family Caecidae. They are micromolluscs found worldwide in warm and temperate seas. Many live in sponges, sandy spots on reefs, or grassy beds in shallow bays and lagoons. Some species are very common, such as the “beautiful caecum” (Caecum pulchellum), but they are easy to overlook because of their tiny size.

The shells are tiny, between 2 and 6 mm long, and range from white to yellowish white, sometimes almost translucent. The adult shell has a curved tube for most of its length; the first stage is spiral-shaped, then it becomes cylindrical. One end is sealed with a permanent calcareous plug, and the other end has a circular, multispiral, horny operculum when the animal withdraws into the shell. The surface is smooth or has many close-set annular ridges.

Caecums feed on single-celled organisms on sand grains or pebbles.