Increases in anti-predation strategies and predator effectiveness during the Phanerozoic have been attributed to evolutionary escalation - or an evolutionary "arms-race" between predator and prey. Numerous crinoids have been recognized with platyceratid gastropods situated over the anus, representing one of the few well-documented parasite-host interactions in the fossil record. Trautschold (1867) hypothesized that crinoids evolved long anal tubes to avoid infestation by platyceratids. This hypothesis could be falsified if the anal tube was a primitive character that evolved once, or was present and secondarily lost in parasitized clades. A phylogenetic analysis of 27 crinoid taxa (Camerata: Compsocrinina) suggests that the anal tube is a derived character that evolved independently at least 4 times in platyceratid-infested clades, failing to falsify the hypothesis that anal tubes evolved in response to parasitism.