New information, from study of deformed columns of the Late Ordovician crinoid Heterocrinus juvenis Hall, 1866, lends support to the hypothesis (proposed by Graff, 1885) that perforated swellings in fossil crinoid columns are myzostomid (annelid) galls. Oval calcium-phosphatic rings, occurring both on column surfaces and inside column galls, resemble Recent encysted myzostomes in size, shape, and orientation. These resemblances, coupled with the similarity of fossil swellings to Recent myzostome galls and the abundance of Recent crinoid infestation by myzostomes provide substantial, although inconclusive, support for the myzostomid affinities of perforated swellings in the columns of fossil crinoids.