Monogenean neuromusculature: some structural and functional correlates

Monogenean neuromuscular systems are structurally and functionally well-differentiated, as evidenced by research on the fish-gill parasite, Diclidophora merlangi. The nervous system in the worm exhibits a raft of putative intercellular signalling molecules, localised in neuronal vesicles. There is cytochemical evidence of co-localisation of neuropeptides and cholinergic substances, with aminergic components generally occupying separate neurons. The phalloidin-fluorescence technique for F-actin has enabled the demonstration of muscle organisation in the worm. Body wall musculature comprises circular, longitudinal and diagonal arrays of myofibres whose contractions are believed to be largely myogenic; circular fibres predominate in the walls of the reproductive tracts. The major somatic muscles are longitudinal muscle bundles that traverse the mesenchyme, the most extensive of which extend from the pharynx to the clamps of the haptor. Experiments have shown that some of these muscles may serve in a withdrawal reflex in the worm, which can be evoked by water turbulence. These and the muscles of the suckers, pharynx, clamps, male copulatory organ and ootype are provided with extensive synaptic innervation that is strongly immunoreactive for FMRFamide-related peptides (FaRPs), suggesting contractions may be neurogenic. Examination of the physiological effects of known flatworm FMRFamide-related peptides on muscle contractility in vitro has shown those FMRFamide-related peptides isolated from turbellarians to be the most excitatory. Results are discussed with respect to neuromuscular function in adhesion, alimentation, and reproduction in the worm.