Structure and ultrastructure of muscles in the priapulid Halicryptus spinulosus : functional and phylogenetic remarks

Somatic muscles (body wall and introvert retractors) and visceral muscles (gut and urogenital apparatus) of Halicryptus spinulosus have been examined under the optical and electron microscope. Somatic muscles show similar morphological features: they are circomyarian fibres with globular or rod-like Z-bodies and highly interdigitated and staggered filaments. Complex membranous systems forming hanks are typical of these muscles. Visceral muscles are made up by fibres with an even more irregular internal pattern: there are no T-systems. The mesenteries supporting the urogenital apparatus show small myoepithelial fibres. Our data is compared with a number of observations of various invertebrate muscles. This comparison shows that Halicryptus muscle fibres are unlike those of other groups, thus lending support to the view that Priapulida is a very primitive and independent group.