The cylichnids possess a strong, external shell that is typically cylindrical, but it is ovate or even globose in some genera. It has a short or depressed spire. The shell is usually plain white in colour and glossy or semitranslucent. Although there are many species, they share a similar shell shape and the majority are less than 5 mm long.
The protoconch (larval shell) is often still conspicuously attached to the tip of the spire. The animal is usually small and unpigmented, usually only about a third of the shell length. Almost all cylichnids are able to withdraw completely into the shell. An operculum is usually absent.
The short head shield possesses a pair of flattened posterior lobes that reach back to the shell to protect the mantle cavity as they lead a burrowing infaunal lifestyle in clean sand. Diet consists of foraminiferans.
The foot is short without parapodia. The gill is not visible externally.
All cylichnids are infaunal, residing just below the surface of the substrate.