1. B. J. Swalla, Department of Biology, Vanderbilt University, Nashville, TN 37235, USA, and Station Biologique, Roscoff, France.
2. W. R. Jeffery, Bodega Marine Laboratory, Bodega Bay, CA 94923, USA; Section of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA; and Station Biologique, Roscoff, France.