The T-axial membrane system in striated muscles of the horseshoe crab.

Cardiac muscle and skeletal muscles powering walking leg, tailspine and gill movements in Limulus polyphemus were studied by transmission electron microscopy. All muscles examined had extensive invaginations of the sarcolemma at the Z disc and at the lateral margins of the A band. Invaginations at the Z disc were often branched in the transverse and longitudinal planes, but branching was not observed at other locations within a sarcomere. Dyads, and occasional triads, were observed at the A band in all muscles examined.