Ultrastructural studies on muscle attachments in Reighardia sternae Dies. (Pentastomida)

Summary

Muscle insertions on to the cuticle of a cephalobaenid pentastomid, Reighardia sternae, are described. These attachments are typically arthropodan in that tendinous epidermal cells, containing tracts of axially orientated microtubules, connect the muscles to the cuticle. The microtubules are associated apically with conical hemidesmosomes from which arise attachment fibres; these fibres are integrated into the substructure of the subcuticle and only attach to the epicuticle during eedysis. In sclerotized areas of cuticle, attachment fibres follow pore canals. Basally the microtubules terminate in a hemidesmosome which interdigitates with a muscle fibre hemidesmosome derived from Z-line material. An innovation in pentastomid muscle attachments is related to the use of the body wall in locomotion; this being effected by parietal circular and longitudinal muscle systems. Precise control of cuticle movements is achieved by the lateral attachment of long tracts of muscle fibres to the cuticle via the tendinous epidermal cells; the latter are modified to be suspensory rather than a strictly mechanical connection (this is usually necessary for muscle activity in other arthropods). Thus in Reighardia, parietal muscle activity is converted into a corresponding movement of the cuticle. The phyletic significance of these observations is discussed.