The blood-system in the Serpulimorpha (Annelida, Polychaeta). II. The anatomy of the blood-system in the Sabellidae, and comparison of Sabellidae and Serpulidae.

I . An account will be given of the anatomy of the blood-system of Pomatoceros triqueter, and of comparative observations on the following serpulids: Serpula vermicularis, S. lo-biancoi, Hydroides norvegica, Vermiliopsis infundibulum, Salmacina incrustans, Protula intestinum, P. tubularia, Apomatus ampulliferus, A. similis, Spirorbis militaris, and S. corrugatus. 2. In all species there is a central blood-system of large vessels in which blood circulates in the usual manner, and a peripheral system of small blind-ending vessels which are alternately full and empty, receiving their blood from the central system, then returning it along the same channels to the central system. 3. The central blood-system is as follows: Blood moves from the tip of the abdomen to the front of the thorax through a sinus enveloping the alimentary canal. Anteriorly it passes through dorsal, transverse, and circum-oesophageal vessels to a ventral vessel in which it moves backwards to die tip of the abdomen. A pair of ring vessels connects the ventral vessel with the sinus at the posterior end of each segment. 4. The anterior end of the dorsal vessel in Pomatoceros triqueter, Serpula vermicularis, Hydroides norvegica, and Vermiliopsis infundibulum is surrounded by a sphincter muscle of unknown function, and contains a muscular valve which probably obstructs the back-flow of blood from the transverse vessel. Protula intestinum possesses the valve but lacks the sphincter. Salmacina incrustans and Spirorbis militaris have neither valve nor sphincter. 5. The peripheral blood-system has the following components: the branchial vessels with branches in the crown; the vessels of the collar and lips; the peri-oesophageal plexuses; the trans-septal vessels supplying the body-wall, parapodia, and thoracic membrane. 6. In Pomatoceros triqueter the opercular vessel is spirally coiled, but in other serpulids it is characteristically branched. 7. When Pomatoceros withdraws into its tube, movement of blood in the crown ceases. The operculum is therefore not used as a special respiratory organ when the crown is retracted. 8. The oesophagus of Pomatoceros is surrounded by two independent blind-ending vascular plexuses, an outer plexus communicating with the gut sinus and an inner plexus with the circum-oesophageal vessels. Serpula vermicularis is probably the same. Hydroides norvegica and Protula intestinum lack the outer plexus. Salmacina and Spirorbis have neither plexus. 9. The body-wall of each segment derives its blood-supply from trans-septal branches of the ring vessels of the preceding segment. In Salmacina and Spirorbis these trans-septal vessels are unbranched. In larger serpulids they have numerous [Quarterly Journal of Microscopical Science, Vol. 91, part 2, June 1950.]