M�LLER, MCM*; Universit�t Osnabr�ck, Germany: Polychaete nervous systems: Ground pattern and variation

In Annelida as well as in other invertebrate taxa the nervous system is considered to be a most conservative organ system. According to the Articulata-hypothesis the specific structure of the rope-ladder-like nervous system is an autapomorphic character that unifies Annelida and Arthropoda. Immunhistochemical investigations in combination with cLSM, however, demonstrate that the annelidan bauchmark, in contrast to the arthropod one, is highly variable regarding number and position of connectives and number of commissures per segment. The variability as well as a hypothesis how it evolved will be introduced with the aid of regeneration- and developmental studies.