The fused thoracic-abdominal ganglion of the hermit crab (Pagurus pollicaris): neuromuscular relationships in the thoracic and abdominal flexor muscles

1. The physiological and morphological properties of the thoracic flexor neuro-muscular systems are described for the hermit crab Pagurus pollicaris. There are two pairs of flexor muscles in thoracic segments 3 to 5: the dorsal flexors and the ventral flexors. 2. The dorsal flexors are composed of fast fibres and are asymmetrical. On the left side the muscle branches laterally so that it has two points of insertion on the thoracic-abdominal sternite, compared with only one for the right side. The ventral flexors are composed of slow fibres. 3. The dorsal flexors are supplied by two excitatory motor axons that run in the thin nerve of the first abdominal root. The ventral flexors are innervated by at least seven excitors: two from the thin nerve of the first abdominal root, three from the thick nerve of the first abdominal root, and two from the thin nerve of the fifth thoracic root. Cobalt backfilling of the first abdominal root revealed that most cells are located on the dorsal and ventral surfaces in the centre of the thoracic-abdominal (TA) ganglion. Although most of the efferent cell bodies from the thin nerve of th