1. Agronomic Centre of Applied Researches of Hainaut, Institute ofAgriculture, rue Paul Pastur 11, B‐7800 Ath, Belgium
2. University Marine Biological Station, Millport, Isle of Cumbrae, Scotland, KA 28 OEG, UK
3. Laboratory of Anatomy, URA 1137 C.N.R.S./Museum, Museum National d'Histoire Naturelle, F‐ 75005 Paris, France