Three types of thick myofilaments in the nephridial muscle cells of the sipunculan Phascolosoma granulatum.

The nephridium muscular layer of the sipunculan Phascolosoma granulatum consists of longitudinal cells oriented in parallel to the nephridial network and of circular cells. These cells are filled with myofilaments and cell organelles, such as mitochondria, nucleus, and sarcoplasmic reticulum; the organelles are located in the peripheral regions of the cells. The thick myofilaments appear to have cross striations at intervals of 14 nm, typical of paramyosin filaments. According to our ultrastructural observations, there are three kinds of thick myofilaments in these cells. The mean diameters are respectively about 28, 42, and 58 nm. The cells can be divided into three types according to the kinds of myofilaments they contain. One type contains all three kinds of myofilaments; the second contains the 28 and 42 nm myofilaments; and the third contains only the 28 nm myofilaments. The fine structure of the cell organelles appears the same in all three types.