Cirral Structure of the Pedunculated Marine Barnacle Lepas anatifera L. (Crustacea, Cirripedia) — I. Ultrastructure of the Neuromuscular Apparatus

Lepas anatifera L. is a common pedunculated marine barnacle with well developed, articulated cirri which gather water‐suspended food particles, aid in predatory food capturing and may facilitate respiratory function by creating water currents through the mantle. The present data are the first ultrastructural findings of the neuromuscular apparatus which supports rhythmic cirral contraction. A single large striated flexor muscle extending the length of the cirus contains mostly one type of myofibril (Type A) but is accompanied by a small band of more compact myofibrils (Type B). A well developed T‐tubular system with extensive sarcoplasmic cisternae and dyads is found, in addition to subsarcolemmal membranous whorls. A central neural complex innervates this muscle and electron‐lucent axons containing large membranous elements and small vesicles and surrounded by supporting cells with extensive rough endoplasmic reticula are present. Intimate association of nerve processes with the flexor muscle consists of neuromuscular junctions with synaptic‐type vesicles, and a well developed mode of sarcolemmal vesicular transport. These focally compacted vesicles communicate directly with the muscle tubular system. Rhythmic movement of cirri is probably controlled by alternating contraction of the flexor muscle followed by relaxation with pulsatile filling of the afferent hemocoel with blood.