How are commissures and connectives arranged in Pila?

The ganglia in Pila are interconnected by nerve cords. These are classified based on whether they connect similar or dissimilar ganglia.

Commissures (Connecting Similar Ganglia)

• Cerebral Commissure: A thick, transverse nerve cord that runs above the buccal mass, connecting the two cerebral ganglia.
• Buccal Commissure: A slender nerve that runs below the oesophagus, connecting the two buccal ganglia.
• Pedal Commissure: A nerve connecting the two pedal ganglia (which are part of the pleuro-pedal masses), running beneath the buccal mass.

Connectives (Connecting Dissimilar Ganglia)

The arrangement of connectives is complex due to an evolutionary event called torsion, where the visceral mass twisted 180 degrees. This results in a figure-eight arrangement known as streptoneury or chiastoneury.

• Cerebro-buccal Connectives: A pair of nerves, each connecting a cerebral ganglion to a buccal ganglion.
• Cerebro-pleural and Cerebro-pedal Connectives: Nerves that connect each cerebral ganglion to its corresponding pleural and pedal ganglia on the same side.
• Pleuro-visceral Connectives (The "Twist"): This is the key feature.
  • The left pleural ganglion connects to the supra-intestinal ganglion (which has moved to the left).
  • This supra-intestinal ganglion then connects to the visceral ganglion.
  • The right pleural ganglion connects directly to the visceral ganglion via a long connective (the infra-intestinal connective) that loops under the intestine.
• Supra-intestinal Nerve: This is a long nerve that crosses over the intestine, connecting the supra-intestinal ganglion (on the left) to the right pleuro-pedal mass. This crossing over the intestine (supra-intestinal) and the other connective running under it (infra-intestinal) forms the characteristic twisted "figure-eight."