Gastropods are characterized by the possession of a single (often coiled) shell, although this is lost in some slug groups, and a body that has undergone torsion so that the pallial cavity faces forwards. They have a well-developed head bearing a pair of cephalic tentacles and eyes that are primitively situated near the outer bases of the tentacles. In some taxa the eyes are located on short to long eye stalks. The mantle edge in some taxa is extended anteriorly to form an inhalant siphon and this is sometimes associated with an elongation of the shell opening (aperture) — this is shown in the photo of the caenogastropod Conus bullatus below. The foot is usually rather large and is typically used for crawling. It can be modified for burrowing, leaping (as in conchs, Strombidae), swimming, or clamping (as in limpets). The foot typically bears an operculum that seals the shell opening (aperture) when the head-foot is retracted into the shell (see photos below). While this structure is present in all gastropod veliger larvae, it is absent in the embryos of some direct developing taxa and in the juveniles and adults of many heterobranchs. The nervous and circulatory systems are well developed with the concentration of nerve ganglia being a common evolutionary theme