The Bikini celebrated its 21st birthday this year--virtually unnoticed. What with milady's hemline going from mid-thigh to just below the crotch (as the microskirt replaces the miniskirt), Paris fashions featuring transparent dresses, banded with beads in the critical zones, and Roman models wearing harem pants slit up to the waist, today's feminine finery has made the bikini about as controversial as the muumuu. The revolution that started when the first crop of bare bosoms popped up (and out) in 1964, after Californian Rudi Gernreich took the top off the swimsuit, proved no flesh in the pan. The suits were soon being worn in public, provoking both cheers and police action. Topless evening gowns put in appearances from London to Hong Kong, and the wave of the future was plain to see. The far-out fall fashions of 1964 featured acres of openwork, often filled in by flesh-colored linings. By December, the linings had given way--in some cases to fish nets, in other cases to flesh. Although Playboy first apprised its readers of the fashion ferment in The Nude Look (November 1965), much has happened since then. Today, with garments of linked metal or plastic disks, a beach belle can easily emulate a patch of water lilies. Do-it-yourself kits permit a creative creature to arrange her own plastic cutouts over a basic dress form of see-through vinyl. Dresses of detachable leather strips allow for exposure up to what the traffic will bare. Some designers have even put the woman outside her clothes, by imprinting the nude female form on the fabric. Couture conservatives, bucking the trend toward minimal styles, were predicting a return to "normalcy." Happily, the Upbeat Generation was proving them poor prognosticators as it continued to jump on the bare band wagon. Unshackling a shapely shape is nothing new, of course; bare-breasted beauties abounded at royal courts from that of ancient Minos to that of Louis XIV. Today's woman, to the delight of males who suffered through the femme-concealing fashions of the Fifties, has rediscovered that sex and style can be synonymous.