ULTRASTRUCTURE OF THE SUBEPIDERMAL MUSCULATURE OF XENOTURBELLA-BOCKI,THE ADELPHOTAXON OF THE BILATERIA

Xenoturbella bocki is the only species of the high-ranked taxon Xenotu rbellida. The species lives on marine mud bottoms at a depth of 20-120 m and moves extremely slowly by ciliary gliding. Nevertheless it poss esses a well-developed body wall musculature with outer circular muscl es, a prominent layer of inner longitudinal muscles and radial muscles that extend from the outer circular myocytes to the musculature surro unding the gastrodermis. The longitudinal myocytes are not compact cel ls, but form fascicles of fibrils running parallel to each other. Fine cytoplasmic cords connect the fibres of a cell to each other and with its nuclear region. The muscles are embedded within a sometimes expan sive extracellular matrix (ECM) that lacks any fibrillar components. A ll muscle cells display conspicuous and numerous cytoplasmic extension s that are intermingled with each other. Tight coupling between adjace nt cell membranes is not found, but zonula adhaerens-like junctions ex ist. Fibrils belonging to different myocytes, but also fibrils of the same cell, are coupled by such cytoplasmic extensions. Circular, radia l and at least the peripheral longitudinal myocytes display cell-matri x connections with the internal lamina, a component of the subepider- mal ECM. This internal lamina projects down into the centres of the fa scicles with longitudinal muscle fibrils and forms extensive attachmen t zones with the muscle cells, reminiscent of focal contacts. For the ingestion of food, X. bocki opens the simple mouth pore and protrudes the aciliated gastrodermis. The body wall musculature is responsible f or this protrusion and also for the withdrawal of the gastrodermis. In the past, possible phylogenetic kinships with the Acoelomorpha (Plath elminthes) or the Enteropneusta and Holothuroidea were discussed, but, on the basis of all information available, X. bocki is hypothesized t o be the sister taxon of the Bilateria.