Adaptation of vermetid and siliquariid gastropods

Most vermetids are cemented to hard substrates, and can extend the shell aperture high above the substrate. The shell can be bent sharply around obstacles, and internal septa allow it to become much longer than the soft parts. Several vermetids use these adaptations to live as facultative soft-bottom dwellers, but no species appears to be permanently adapted to this environment. In contrast, siliquariids are not cemented. Siliquaria lives embedded within sponges as a commensal or parasite, letting its exhalant current flow out of the shell and into the host sponge through a very long slit. Siliquaria is unique among gastropods in that it periodically increases the width of the shell slit and/or the cross sectional area of the whorl by cracking the whorl periphery into wedges, pushing them outward, and subsequently reattaching them together.