Arc Circular

Arc of a circle. Its unit of measure is the degree, = of the circumference. The degree (°) is divided into 60 minutes ('), the minute into 60 seconds ("). The arc of 90° is a quadrant. The arc is also measured in terms of (-3.14159+) which is the ratio of the semi circumference to the radius. As angles at the center of a circle are proportional to the intercepted arcs, arcs are taken as the measures of angles. Arcellina, or TESTACEA. Order of Lobosa, characterized by having the sarcode body enveloped by a horny shell (ar IMG:content-0229.png:[ocr errors][merged small] cella), or one made of grains of sand cemented together (difflugia). There is one opening to this shell, through which the club-shaped pseudopodia are thrust out in search of food. Reproduction is usually by simple fi ion, one fi ion half of the original animal retaining the old shell; the other half, or daughter cell, secretes its own new shell.