Ultrastructure of the dorsal hemal vessel in the sea-cucumber Parastichopus tremulus (Echinodermata: Holothuroidea).

The dorsal hemal vessel in Parastichopus consists of three distinct layers: An outer flagellated epithelium, an intermediate circular muscle layer and an inner connective tissue layer which nearly fills the lumen. Between the outer and intermediate layer runs strands of nerve fibers. Each coelomic epithelial cell has one flagellum and some microvilli. It contains a number of different vacuoles and a few bundles of tonofilaments. One special type of vacuole which contains well organized myofilaments is described. Each muscle cell contains one myofibril of a non-striated type consisting of thick and thin filaments and no dense bodies. The sarcoplasmic reticulum is poorly developed, but peripheral coupling are frequently found. The muscle cells in the dorsal hemal vessel of Parastichopus are compared with other muscles in echinoderms and muscle types described in other phyla.