A Free-crawling young animal. B Slightly-squeezed, mature animal C. Organization scheme of a squeezed animal. D. Bursal pieces on a squeezed animal. E. Organization of the locomotory muscles of the proboscis form the right hand side. F. Terminal organ next to the proboscis. G. Copulatory organ on a squeezed animal. H. Crawling animal of the North Sea coast. I and K. Bursal pieces of squeezed animals from the North Sea coast. - A, B, C, E, F and H freehand, D, G, I and K with camera lucida. D and G 1110x , I and K 1060x