Summary
The somatic muscles of the tardigrades Macrobiotus hufelandi and Milnesium tardigradum consist of elongated cells which are structurally independent of each other and insert in the body wall. The contractile part of the muscle cells contains thick and thin myofilaments. The thin filaments run together in typical dense bodies. In relaxed muscles clear A-, I- and H-zones are formed, and the dense bodies are often accumulated in groups. In ribbon like I-zones they are sometimes arranged side by side in rows and resemble fragmented Z-rods. The sarcoplasmic reticulum is localized primarily in the small region between the contractile apparatus and sarcolemma, and is represented by a system of smooth, flat, reticulated cisternae. The contractile apparatus of the somatic muscles is compared to that of smooth and obliquely striated muscles of several invertebrates. On the basis of certain distinguishing features of the contractile part of somatic muscles of tardigrades, it is suggested that these muscles should be referred to as a type intermediate between smooth and obliquely striated muscles.
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Walz, B. The fine structure of somatic muscles of tardigrada. Cell Tissue Res. 149, 81–89 (1974). https://doi.org/10.1007/BF00209051
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DOI: https://doi.org/10.1007/BF00209051