Ultrastructure of transrectal coelomoducts in the sea cucumber Parastichopus californicus (Echinodermata,Holothuroida)

Summary The perivisceral coelom of the sea cucumber Parastichopus californicus is connected to the lumen of the hindgut by as many as 200 short transrectal ducts. Each duct is lined by a pseudostratified epithelium composed of: (i) monociliated, tonofilament-containing cells, (ii) myoepithelial cells, (iii) bundles of neurites, and (iv) granule-containing cells. In most places the lumen of each duct is lined by the monociliated, tonofilament-containing cells. The myoepithelial cells are predominantly basal in position and circular in orientation, but some border the lumen and parallel the long axis of the duct. The epithelium of a duct consists of the same types of cells as occur in the peritoneum covering the rectum and differs markedly from the nonciliated, cuticularized epithelium that lines the lumen of the rectum. Based on ultrastructural characteristics, the transrectal ducts represent evaginations of the peritoneum overlying the rectum and are thus coelomoducts sensu Goodrich. The possibility is discussed that perivisceral coelomoducts of holothuroids function in regulating coelomic volumes.Abbreviations AE adluminal epithelium - AF anal fold - ANC anal coelom - AS anal sphincter muscle - B bacterium - BL basal lamina - BW body wall - CC coelomocyte - CI cilium - CO collagen fibers - CT connective tissue - CTE ciliated, tonofilament-containing epithelial cell - D desmosome-like junction - FB fibroblast - GB Golgi bodies - GC axon-like process of granule-containing cell - HD hemidesmosome - IJ intermediate junction - INT intestine - LM longitudinal muscles of body wall - LRW luminal surface of rectal wall - ME myoepithelial cell - ML microlamellae - MY myelin-like material - NE neurite - NV nerve - NU nucleus - OI opening of intestine into rectum - PC perivisceral coelom - PT peritoneum - PTF papilliform tube feet - RW rectal wall - RL rectal lumen - RS rectal suspensor - RT respiratory trees - SJ septate junction - SO soma of adluminal epithelial cell - SM subepidermal muscle - TD transrectal duct - TF tonofilaments - WVC lateral water vascular canals