We are greatly indebted to our colleague Richard Hoare (Bowling Green State University, Ohio) for his critical reading and corrections of the English manuscript. This paper is part of the project BTE 2006-04554 of the Spanish Dirección General de Enseñanza Superior e Investigación Científica. The photographs were taken in the Servicios Comunes de Investigación of the University of Oviedo. The useful comments and suggestions of the reviewers (Helga Groos-Uffenorde, Göttingen, and an anonymous reviewer) greatly contributed to improve this paper. Finally, thanks are also given to Silvia Blanco-Ferrera for her help with the illustrations.