A circumesophageal or circumpharyngeal nerve ring is an arrangement of nerve ganglia around the

esophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

of an animal. It is a common feature of nematodes, molluscs, and many other invertebrate animals, though it is absent in all vertebrate animals and is not structurally possible in simpler ones such as

water bear Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...

s. The nerve ring, also called a nerve collar, creates a complete and closed loop around the food-entry parts of the animal's anatomy. In a typical molluscan arrangement, these include the cerebral, pedal, and pleural ganglia, with the esophagus passing through the center of the ring. In nematodes, the ring consists of only two to four large associative cells connected to two paired lateral ganglia, two ventral ganglia, and a single unpaired dorsal ganglion. Among arthropods, the usual arrangement is a single ganglion (the cerebral) positioned above the esophagus, a single ganglion or nerve mass (the subesophageal) below it, and commissures connecting the two in a ring. Among the gastropods, the evolutionary

torsion Torsion may refer to: Science * Torsion (mechanics), the twisting of an object due to an applied torque * Torsion of spacetime, the field used in Einstein–Cartan theory and ** Alternatives to general relativity * Torsion angle, in chemistry Bi ...

event which relocates the anus to near the head of the animal and allowing it to withdraw into its shell has relocated the commissure of the pleural ganglia into a "twist" (the right ganglion has relocated to the left side, and the left ganglion to the right).

References

{{reflist Arthropod anatomy Mollusc anatomy Animal nervous system