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 neuromuscular 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 the fifth thoracic root are located rostrally in the midline of the TA ganglion, two or three somata can be found in the same region as cell bodies from the first abdominal root. 4. Stimulation of the giant interneurone elicits a spike in the giant flexor motor neurone and a contraction of the dorsal flexor muscles, and a spike in one of the axons in the thin nerve of the fifth thoracic root and a contraction of the ventral flexors. 5. Axons in the third abdominal root innervate the flexor muscles in the first abdominal segment. Motor axons in this root, which includes a giant flexor motor neurone, have cell bodies in the TA ganglion — not in the first abdominal ganglion. 6. Most of the motor axons in the fourth and fifth thoracic roots have cell bodies located in the fused TA ganglion. This suggests that the TA ganglion consists of a fusion of the first abdominal ganglion and the fourth and fifth thoracic ganglia.