The muscle of a monogonont rotifer, Trichocerca rattus. II. The central retractor muscles.

This work concerns the ultrastructural characteristics of the two cells of the paired, bicellular, central retractor muscles (CRM) of the rotifer Trichocerca rattus. By transverse ultrathin serial sections, precise measurements where made on the following cytological characteristics: the length, diameter and average number by unit surface area for the myofilaments; the percentage of cell volume for the myoplasm and mitochondria; and the average number of peripheral elements of the sarcoplasmic reticulum (dyads) by unit surface area. These characteristics agree with a phasic fibre type for the CRM. The muscular insertions are described, as are the junctions between the two cells of each CRM: hemidesmosomes, desmosomes and gap junctions. The two symmetrical CRM are coupled by a large gap junction between two subcerebral sarcoplasmic processes. Some of these processes partially enter the cerebral neuropile; each CRM is innervated at this level. The functions of the cytological specializations are discussed in the light of the behaviour assumed by the CRM: the retraction of the rotatory apparatus in the trunk of T. rattus.