Functional morphology of vibratile urnae in the synaptid holothuroid Leptosynapta inhaerens (Echinodermata)

Summary

Vibratile urnae of Leptosynapta inhaerens are organized in three longitudinal bands along the mesenteries. An individual of 10 cm length usually houses about 4500 urnae. These are minute (300 Gmm high), S-shaped and hollow peritoneal organs consisting of an intracoelomic projection of the body wall mesothelium supported by a thin connective tissue layer. The urnal cavity is strongly ciliated. Each urna harbours a clump of coelomocytes at the lower part of its aperture. The clump is attached to the urna through spot-like desmosomes occurring between its inner-most coelomocytes and apical urnal cells. Clumps and urnae form functional units. Urnal cilia produce steady water currents through urnal cavities and whirls along urnal bands. The particulate material conveyed by the coelomic fluid enters the urnal cavity and is either trapped by coelomocyte pseudopodia or agglutinated by a mucoid substance that covers the clump's outer surface. Depending on individuals, clearance of coelomic fluid occurs from 2 to 3 h after experimental injection of particulate material. The effectiveness of coelomic fluid clearance appears to be due to the particular organization and location of urnae, viz. in longitudinal bands along the mesenteries.