The authors wish to thank H.A. Wagemaker for his technical assistance and the staff from the aquarium for taking care of the limpets. Last but not least, Mr. W.J. Hage is thanked for his assistance with the CLSM. P.D. was supported by the Earth and Life Sciences foundation (ALW), which is subsidized by the Netherlands Organisation for Scientific Research (NWO), the Netherlands. This is publication number 19 of the Dutch national research program in evolution and development, grant num. 805.33.430.