The fine structure of the somatic muscles and their attachment to the cuticle in the pyenogonidsNymphon (Chaetonymphon) macronyxG. O. Sars andBoreonymphoncf.abyssorum(Norman) is described. The muscles possess characteristics which are typical of arthropod slow muscle fibers: relatively long sarcomeres, a mean A‐band length of about 6 μm and a ratio of thin to thick contractile filaments of 4:1. The sarcotubular system consists of distinct t‐tubules, an irregular SR part and randomly distributed dyads and triads, the muscles are attached to the cuticle by specialized epidermal cells containing microtubules extending from the cuticular to the muscular side. The myoepidermal and epidermal‐cuticular junctions are des