COMPARATIVE BODY-WALL MUSCULATURE AND MUSCLE-FIBER ULTRASTRUCTURE IN BRANCHIOBDELLIDANS (ANNELIDA, CLITELLATA), AND THEIR PHYLOGENETIC SIGNIFICANCE

The body wall muscles in five species of branchiobdellidans are all ar ranged in the oligochaete pattern and the muscle fibres are obliquely striated. The structure of the circular muscle fibres do vary to some degree. The longitudinal muscle fibres in Ankyrodrilus legaeus, Branch iobdella kozarovi, and Xironogiton instabilis all are round circomyari an and thus double-obliquely striated. These species represent three o f the four genera composing the family Branchiobdellidae. Although Bde llodrilus illuminatus and Cambarincola fallax, from the families Bdell odrilidae and Cambarincolidae, respectively, also possess a few round circomyarian fibres, most are 'polyplatymyarian' comparable to single- obliquely striated fibres. A similar division of branchiobdellidan fam ilies is obtained based on the number of anterior nephridial pores. Th e muscular structure in the branchiobdellidans shows both similarities and differences with the leeches and the lumbriculid oligochaetes. On e phylogenetic explanation for this is that the branchiobdellidans sep arated from the common clitellate ancestor before the oligochaetes and leeches became recognizable taxa.