DURING an investigation of the blood system in polychaetes of the families Serpulidae and Sabellidae the musculature of the alimentary canal has been studied. In the following species the only muscle coat, which consists of fibres lying transverse to the main axis, lies outside the blood sinus which envelops the canal: Serpula vermicularis L., Hydroides norvegica (Gunnerus), Vermiliopsis infund...