A model of the structure and evolution of cometary rays is described which considers the rays to be the channeled outflow of cometary ions from the ionosphere. Ions in the central tail ray flow down the channel formed by the tailward closing of the draped interplanetary field. Ions in the side rays flow through magnetic flux ropes which become embedded in the cometary ionosphere through Kelvin-Helmholtz instability of the cometary ionopause. The flux ropes tunnel through the ionosphere, penetrate the ionopause at two points, and extend out into the solar wind on both sides of the comet. As the flux ropes are dragged through the ionosphere by the solar wind, they are continually resupplied with ionospheric plasma through Bohm diffusion driven by ion-neutral collisions. Differences in observed cometary ray structure are attributed to differences in the radial distance and ecliptic latitude of different comets at the time of observation, as well as to variations in the interplanetary magnetic field direction and the gas production rate of the comet.