Different marine invertebrates have different tubular or vesicular systems within their locomotor muscle fibres. The siphonophores Chelophyes, Abylopsis and Muggeia have invaginated tubules which are the morphological equivalent of the vertebrate invaginated tubular system, but lack a sarcoplasmic reticulum. In Chelophyes the previous suggestion that Ca2+ channels in the extensive invaginated tubule system allow ingress of Ca2+ is shown to be incorrect. Contraction of the swimming muscles in Chelophyes is not blocked by 20 μM ryanodine, nor is it induced by 10 mM caffeine, hence intracellular Ca2+ stores appear absent. Contraction is, however, maintained by replacement of the greater part of the usual external Na+ by Li+ or by or N-methyl-D-glucamine, although action potentials can still be evoked. Hence we conclude that following contraction, internal Ca2+ is reduced by a Na/Ca2+ exchange mechanism.