THE HISTOLOGY OF THE BLOOD SYSTEM IN OLIGOCHAETA and POLYCHAETA
Authors: HANSON JEAN 
Published: January 2008

Summary1. A survey of accounts of the histology of the blood system in the various families of the Oligochaeta and Polychaeta has revealed a surprising lack of agreement among descriptions not only of the same family, but often of the same genus or species. There are several reasons for this. Many annelid vessels are very small and are difficult objects for histological investigation. They have rarely been examined in detail whilst alive. Few people have employed special staining methods for distinguishing connective tissue fibres from muscle fibres; nearly always both are present, and they have often been confused with each other. The lack of any comprehensive and analytic review of the scattered literature on the subject seems to be one of the reasons for many of the discrepancies.2. Not only is there disagreement in factual descriptions of histological structure, but, since 1904, there has also been much discussion about the interpretation of observations, occasioned by two important rival theories of the origin of the annelid blood system (Lang, 1904; Vejdovsky, 1905).3. Lang postulated that the haemocoel occupies the site of the blastocoel. The gut sinus is bounded internally by the basement membrane of the gut epithelium. The outer wall of the sinus and the walls of the vessels are derived from splanchnopleuric mesoderm, and consist of the peritoneum and its basement membrane (Leydig's ‘Intima’). An endothelium, where present, is a secondary component of the walls; its origin is obscure.4. Vejdovsky considered that the gut sinus is intra‐endodermal. The blood bathes the gut epithelium, the basement membrane of which is situated in the outer wall of the sinus. This outer wall, and the walls of the blood vessels, have both endodermal and mesodermal components. The latter is the peritoneum. The endodermal component consists of a skeletal coat (vasothelial membrane, equivalent to Leydig's intima) and vasothelial cells of endodermal origin. These cells, corresponding to the endothelium of other authors, are primary components of the walls.5. It has been shown that Vejdovsky's observations disagree with those of previous workers, and that later research has not confirmed them. They are open to criticism because he used inadequate staining methods.6. The theories of Lang and Vejdovsky led to controversy in the interpretation of observations. They also stimulated a large amount of research, much of which was uncritical. For example, two adherents of Vejdovsky's theory noticed a basement membrane on the gut epithelium, but reiterated the statement that the sinus is intra‐endodermal. Similarly, many supporters of Lang's theory paid little attention to cells lying on the inner surface of the skeletal coat, because Lang considered that they are secondary components of the wall; they have frequently been dismissed as sessile blood corpuscles.7. An analysis of all the literature on the histology of the blood system in the Oligochaeta and Polychaeta has led to the following conclusions, which should be regarded as tentative until more careful and detailed observations have been made.8. The walls of the blood system contain a homogeneous connective tissue membrane. This skeletal coat is covered by a peritoneum which is differentiated in various ways in different vessels. On the inner surface of the skeletal coat lies a discontinuous endothelium.9. The endothelium probably often takes the form of branched cells joined together by their processes. Endothelial cells are flat, and their flat oval nuclei are elongated in the direction of the length of the vessel. Fibrils have sometimes been found in the cells, but have never been shown to be contractile. However, there is evidence that the endothelium may in some cases be contractile. Iron and chloragosome‐like bodies have occasionally been found in it.10. The skeletal coat is a continuous homogeneous membrane of a collagenous substance. It is linked by fibres with connective tissue outside the vessel. It becomes longitudinally folded when the vessel contracts.11. The peritoneum is differentiated in various ways, probably correlated with the size and contractility of the vessels. It may be a simple flat epithelium without fibrils; an epithelium with intracellular muscle fibrils; a muscle‐epithelium; a coat of muscles with a cellular covering consisting both of peritoneal epithelial cells and of the cell bodies of the muscle fibres; a coat of muscles covered by a complete peritoneal epithelium.12. The muscle fibres are of the nematode type, i.e. they consist of a cell body containing the nucleus, and processes containing the muscle fibrils. The nucleus is large and nearly spherical and has a conspicuous nucleolus. On small vessels the muscle cells are sometimes stellate, resembling the Rouget cells of vertebrate capillaries. Like Rouget cells, they can be vitally stained with methylene blue.13. Various descriptions of the internal structure of the larger muscle fibres have been given. Some are apparently unstriped. Others have a double oblique striation. Others are said to show a true cross‐striation. In some cases the fibre apparently contains both striped and unstriped fibrils.14. Several theories of the origin of the various coats in annelid vessel walls have been advanced (notably by Lang and Vejdovsky). Evidence based on comparative anatomy, embryology and studies on regeneration suggests that Lang's theory is to be preferred. The haemocoel occupies the site of the blastocoel. The walls of the blood system are entirely mesodermal. The walls of the larger vessels, and the outer wall of the gut sinus are formed by the splanchnopleuric mesoderm, to which belong the peritoneum and the skeletal coat. Nothing is known about the origin of the endothelium.I wish to acknowledge the help I have gained by many discussions of this subject with Dr A. Stock. I am grateful to him and to Dr B. M. Walshe for reading and criticizing the manuscript.