Trochidae

The Trochidae, common name top-snails or top-shells, are a taxonomic family of very small to large sea snails, marine gastropod molluscs in the clade Vetigastropoda (according to the taxonomy of the Gastropoda by Bouchet & Rocroi from 2005). This family is commonly known as the 'top-snails' because in many species the shell resembles a toy spinning top. The animal is similar in general form to the Turbinidae. The top snails are characterized by some primitive traits: a heart with two atria. They have retained only one kidney and the second osphradium has been lost in the course of evolution. The mantle cavity contains a single gill. Along the side of the foot are three or more pairs of sensory epipodial tentacles. The head has a short, broad rostrum. The intertentacular lobes are simple or digitated, separate or united across the front, sometimes obsolete. The jaws are developed or absent. The radula is rhipidoglossate. The rhachidian teeth are always present and well-developed. The lateral teeth generally number 5 on each side, sometimes more numerous. The marginal teeth are narrow and very numerous. The length of an adult shell varies between 5 mm and 130 mm. There is also a wide variation in the shape of the shell. This goes from low auriform (ear-shaped) with a wide aperture to the long, slender conical forms of typical top shells. The shape may also be subglobose, turbinate or helicoid. Their height may vary between 3 mm and 152 mm. The shell contains only a few whorls. These have a highly variable exterior, ranging from smooth or glossy to sculptured. The internal shell is nacreous. They have a brown, entirely corneous, circular, multispiral operculum which fits the aperture snugly. The operculum is formed of numerous gradually increasing whorls with a central nucleus. The aperture may be entire, tetragonal or rounded and has no reflected lip. The peristome is generally not continuous.