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Sinistral and dextral are scientific terms that describe chirality (“handedness”) or relative direction in a number of disciplines. The terms are derived from the Latin words for “left” (sinister) and “right” (dexter). Other disciplines have different terms (such as dextro- and laevo-rotary, in chemistry, or clockwise and anticlockwise in physics) or simply use left and right (as in anatomy).
It is important to note that relative direction and chirality are distinct concepts. Relative direction is from the point of view of the observer. A completely symmetrical object has a left and a right side, from the observer's point of view, if the top and bottom and direction of observation are defined. Chirality, however, is observer-independent: no matter how one looks at a right-hand screw thread, it remains different from a left-hand screw thread. Therefore, a symmetrical object has sinistral and dextral directions arbitrarily defined by the position of the observer, while an object that has chirality can have sinistral and dextral directions defined by characteristics of the object, no matter the position of the observer.
- Main article: Gastropod shell
Because the coiled shells of gastropods are asymmetrical, they possess a quality called chirality, the "handedness" of an asymmetrical structure.
Over 90%  of gastropod species have dextral (right-handed) shells in their coiling. A small minority of species and genera are almost always sinistral (left-handed). A very few species show an even mixture of dextral and sinistral individuals (for example, Amphidromus perversus).
- Main article: Flatfish
The most obvious characteristic of flatfish, other than their flatness, is asymmetry because both eyes are on the same side of the head in the adult fish. In some families of flatfish, the eyes are always on the right side of the body (dextral or right-eyed flatfish), and in others, they are always on the left (sinistral or left-eyed flatfish). Primitive spiny turbots include equal numbers of right- and left-sided individuals, and are generally more symmetrical than other families.
- ↑ Schilthuizen M. & Davison A. (2005). "The convoluted evolution of snail chirality". Naturwissenschaften 92(11): 504–515.
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- ↑ Amphidromus perversus (Linnaeus, 1758)
- ↑ Chapleau, Francois & Amaoka, Kunio (1998). Paxton, J.R. & Eschmeyer, W.N. Encyclopedia of Fishes, xxx, San Diego: Academic Press.