1. The Laboratory, Marine Biological Association, Citadel Hill, Plymouth, U.K.; Department of Cellular and Comparative Biology, State University of New York at Stony Brook, Stony Brook, N.Y. 11790, U.S.A.
2. The Laboratory, Marine Biological Association, Citadel Hill, Plymouth, U.K.; Laboratoire de Neurobiologie Cellulaire, Centre Nationale de la Recherche Scientifique, 91190 Gif-sur-Yvette, France
3. The Laboratory, Marine Biological Association, Citadel Hill, Plymouth, U.K.; Department of Zoology, University of Cambridge U.K.