PLANARIA, in natural history, a ge nus of the Vermes Intestina class and order. Generic character : body gelatin ous, flattish, with a double ventral pore ; mouth terminal. There are about fifty species, divided into six sections, distin guished by the number of their eyes : A without eyes : B with a single eye : C with two eyes : D with three eyes : E with four eyes : and F with numerous eyes. Of the first division we may notice, P. quaclrangularis ; body pale, ovate, very sharp-pointed before, and winged with small curled longitudinal membranes.— It is found in Europe in ditches among duck-weed ; very soft, pellucid, of a changeable form, and moves like a slug, leaving a slime on the bodies it passes over ; when it meets another animal it. draws itself in like a snail.
Of the third division we have a species which is very common in Pennsylvania, in habiting running waters, large creeks, &c. It may be found by turning up the stones which lie in the water, and looking atten tively on their under surface. We have named this species P. triangularis ; bo dy pale, linear, rounded behind, head triangular, angle in front acute, lateral acute angles extending rather beyond line of the body, eyes round, black, conspicuous, placed about the middle of the head, and partly surrounded each with a whitish mark or lunule; this, how ever, is sometimes obsolete. It trails
along with a slow but regular gait, over the stones, plants, &c. in the water, and never ventures out of it. Length about three-tenths of an inch.
l'LANE, in geometry, denotes a plain surface, or one that lies evenly between its bounding lines : and as a right line is the shortest extension from one point to another, so a plain surface is the shortest extension from one line to another. In astronomy, conics, &c. the term plane, is frequently used for an imaginary sur face, supposed to cut and pass through solid bodies ; and on this foundation is the whole doctrine of conic sections built. See Come sections. ' In perspective, we meet with the per spective plane, which is supposed to be pellucid, and perpendicular to the hori zon ; the horizontal plane, supposed to pass through the spectator's eye, parallel to the horizon ; the geometrical plane, likewise parallel to the horizon, whereon the object to be represented is sup posed to be placed, &c. See PERSPEC TIVE.
The plane of projection, in the stereo graphic projection of the sphere, is that on which the projection is made ; cor responding to the perspective plane.