Clot formation in the sipunculid Themiste petricola, a coelomate nonsegmented marine worm without a circulatory system, is a cellular response that creates a haemostatic mass upon activation with sea water. The mass with sealing properties is brought about by homotypic aggregation of granular leukocytes present in the coelomic fluid that undergo a rapid process of fusion and cell death forming a homogenous clot or mass. The clot structure appears to be stabilized by abundant F-actin that creates a fibrous scaffold retaining cell-derived components. Since preservation of fluid within the coelom is vital for the worm, clotting contributes to rapidly seal the body wall and entrap pathogens upon injury, creating a matrix where wound healing can take place in a second stage. During formation of the clot, microbes or small particles are entrapped. Phagocytosis of self and non-self particles shed from the clot occurs at the clot neighbourhood, demonstrating that clotting is the initial phase of a well-orchestrated dual haemostatic and immune cellular response. 1. Introduction Sipunculans are a phylum of nonsegmented peanut-shaped marine worms that lack a true circulatory system [1]. Recent molecular phylogenetic analyses suggest a close relationship between Sipuncula and the phylum Annelida, particularly with the major group Polychaeta that includes mostly marine worms [2–4]. These worms have a coelomic cavity filled with cells in suspension enclosed by a muscular body wall (Figure 1). The coelomic cavity serves as a hydroskeleton and is lined by a peritoneum, surrounded by a muscular layer, a dermis, an epidermis, and a cuticle [5]. In some species the coelomic cavity has a series of canals that penetrate the body wall toward the dermis while in other species these canals form an interconnected network providing a comprehensive system for coelomic fluid circulation [1, 5]. Figure 1: Themiste petricola, a species of the phylum Sipuncula, is shown in “peanut-shape” in (a) and in “worm-shape” in (b). Shape changes are due to contraction of the retractor and longitudinal muscles in (a), and relaxation of retractor and contraction of circular body wall muscles in (b). Intracoelomic pressure is higher in peanut-shape, and coelomic fluid may be strongly expelled if the body wall is ruptured while the worm is kept in this shape. Although several studies of coelomic cells have been conducted in species of Sipuncula for more than a hundred years, the presence of a clotting system has not been demonstrated until recently [6]. Comprehensive reviews on the phylum [1, 5]
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