Nematocytes rapidly emigrate from normal tissue implanted into an epithelial hydra, which lacks its own nematocytes. We observed emigrating nematocytes, both in time-lapse movies and in fixed whole mounts, to learn what controls directionality. The cells migrate at speeds of 10--30 microns/min with a pseudopod forward and the capsule trailing. Migration is aligned parallel to the polyp axis, but cells move both proximally and distally and often shuttle back and forth. Thus, migration is strongly bidirectional, with a slight distal bias since at any time more cells are moving towards the tentacles. Nematocytes freely migrate through tissue of reversed polarity. No evidence was found for a chemotactic stimulus emitted from the tentacles. The bidirectional movement pattern appeared to be imposed by epithelial muscle processes, which form an axial, fibrous mat on which nematocytes migrate. Experimental deformation of the muscle alignment results in a corresponding deformation of nematocyte migration. Thus, nematocyte migration appears to be bidirectionally oriented through contact guidance by epithelial muscle processes.