Internal Anatomy: A typical animal belonging to the groups handled here can be divided into two equivalent halves by a line passing through the endocoels between the directive mesenteries. This line is the directive axis. In order to describe the position of the 8 primary mesenteries one uses the arbitrary terms dorsal and ventral for the two directive couples and dorso-lateral and ventro-lateral for the lateral ones. The ventral directive couple is the one towards which the retractors on the four lateral mesenteries face.