ABSTRACT
The histology of the blood-system of Pomatoceros triqueter has been studied in detail. Comparative observations have been made on the following serpulids and sabellids: Serpula vermicularis, Hydroides norvegica, Vermiliopsis infundibulum, Sal- macina incrustans, Protula intestinum, Apomatus ampulliferus, Spirorbis militaris, S. cor- rugatus, Sabella spaUanzanii, Potamilla sp., and Dasychone lucullana. Living coelomic capillaries of Sabella have been investigated.
All vessels possess a three-layered wall consisting of an endothelium, a skeletal coat, and a peritoneum containing muscle-fibres which lie transversely to the long axis of the vessel.
The outer wall of the gut sinus had the same three layers. On its inner side the sinus is bounded by an endothelium lying on a skeletal coat which in most species is the basement membrane of the gut epithelium. In Sabella and Branchiomma a muscle coat, with fibres lying transversely to the long axis of the gut, is situated inside the inner skeletal coat and rests on the basement membrane of the gut epithelium. The function of these muscles is not known. Longitudinal muscles are found in the same position in Myxicola.
The endothelium of the coelomic capillaries of Sabella, and probably of all serpulids and sabellids, is a syncytial reticulum of cell bodies connected by cytoplasmic strands.
The endothelium of Sabella and Branchiomma, but not of the other species investigated, contains chloragosome-like globules. In Sabella it also contains iron in some organic compound.
Blood-cells are absent from all serpulids and sabellids investigated.
The skeletal coat is linked by fibres with the rest of the internal skeleton of the body. It is a homogeneous sheet of a substance giving the staining reactions of collagen. Reticular and elastic fibres are absent from it. When the vessel contracts the skeletal coat does not change in thickness but is thrown into longitudinal folds.
The coelomic capillaries of Sabella have a peritoneum which is apparently syncytial, and in which are muscle-fibres arranged in a wide-meshed reticulum. The reticulum and the nuclei of the peritoneum can be vitally stained with methylene blue. The nuclei are sparsely scattered over the surface of the vessel without any special relation to the fibres of the reticulum.
On the larger vessels and on the gut sinus separate muscle-fibres, each with one nucleus, are present. The peritoneum constitutes a muscle-epithelium. The nucleus lies in a small membrane of cytoplasm extending along the outer surface of the fibre.
The muscle-fibres of the gut sinus are composed of unstriped fibrils. The fibres of the smaller vessels sometimes show alternating stained and unstained bands of equal length.
On the larger vessels each muscle-fibre apparently contains both striped and unstriped fibrils. The fibre seems to be covered by a thin sheath of striped fibrils covering a central core of unstriped fibrils which are arranged so that the core shows a double-oblique striation.
The muscles of the rest of the body are unstriped.
