Diagnostic characters
Differs from the previous species in that the tube is marked only by incon¬spicuous rings, in that the blind end is closed by a rounded plate, and in being smaller.

Other characters
C. glabrum has the same mode of life as C. imperforatum. It is more wide¬spread, both within the British Isles and in extending to Norway and Sweden. The animals are summer breeders, laying transparent spherical capsules in the sand on which the adults live. There is one egg in each capsule which hatches to a veliger larva with a planospiral shell of 1-2 whorls. Its bilobed velum has a marginal band of purplish black. At a later stage the last whorl begins to grow away from the spire and still later, after the larva has settled and metamorphosed, the larval shell is broken off leaving only the last whorl, the upper end of which becomes sealed. With growth the oldest part of the tube is broken off, the hole again sealed so that the shell never appears to change shape (Gotze, 1938).
What was once regarded as a variety of this species has recently been accepted as distinct. This is C. armoricum de Folin, 1869, which has been found in Brittany and Dorset. The shell may be distinguished from that of C. glabrum by the possession of a more prominent and asymmetrically pointed septum closing the shell.