Organization of Primitive Nervous Systems. Neuromuscular Physiology Ofgyrocotyle Urna,a Parasitic Flatworm

The nervous system consists of two longitudinal nerve cords with commissures at each end. Ganglia are reduced to small swellings in the cords adjacent to the commissures. At the posterior end there is a nerve ring with a reduced nerve plexus in the rosette. 2. The responses to electrical stimulation are described. Preparations have high thresholds and relatively slow twitch contractions. 3. Direct stimulation often causes loss of muscular tone. This and decreased responsiveness to repeated stimulation are thought to be due to the presence of inhibitory neurones. 4. Facilitation of the response to electrical stimulation is short lasting. This effect has usually decayed within a few seconds of stimulation. 5. Rhythmical spontaneous contractions occur. Pacemakers for this activity are probably in the longitudinal nerve trunks. 6. The anterior acetabular commissure is used to synchronize the spontaneous activity of the two sides. Electrical stimuli applied to one side will cause the other to contract if the anterior commissure is intact.