PHALLOIDIN-RHODAMINE PREPARATIONS OF MACROSTOMUM-HYSTRICINUM MARINUM (PLATHELMINTHES) - MORPHOLOGY AND POSTEMBRYONIC DEVELOPMENT OF THE MUSCULATURE

A whole-mount fluorescence technique using rhodamine-labeled phalloidi n was used to demonstrate for the first time the whole muscle system o f a free-living plathelminth, Macrostomum hystricinum marinum. As expe cted, the body-wall musculature consisted of circular, longitudinal, a nd diagonal fibers over the trunk. Also distinct were the musculature of the gut and of the mouth and pharynx (circular, longitudinal, and r adial). Dorsoventral fibers where restricted in this species to the he ad and tail regions. Circular muscle fibers in the body wall were ofte n grouped into bands of up to four parallel strands. Surprisingly, dia gonal fibers formed two distinct sets, one dorsal and one ventral. Cer tain diagonal muscle fibers entered the wall of the mouth and were con tinuous with some longitudinal muscles of the pharynx. Dorsoventral fi bers in the rostrum occurred partly in regularly spaced pairs, a fact not known for free-living Plathelminthes. All muscle fibers appeared t o be mononucleated. During postembryonic development, the number of ci rcular muscle fibers can be estimated to increase by a factor of 3.5 a nd that of longitudinal muscles by a factor of 2. Apparently as many a s 700-800 circular muscle cells must be added in the region of the gut alone during postembryonic development. Stem cells (neoblasts), ident ified by TEM in the caudalmost region of the gut, lie along the latera l nerve cords. In the same body region most perikarya of circular musc le cells occurred in a similar position. This suggests that the nucleu s-containing part of the cell remains in the position where differenti ation starts.