Streptoneury or chiastoneury is a

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

condition present in all gastropods which is a result of an evolutionary event called torsion in which the intestines, heart, nephridia, gills, and nerve cords "twist" causing some organs to migrate from the animal's left to its right in order to accommodate the relocation of the

mantle cavity The mantle (also known by the Latin word pallium meaning mantle, robe or cloak, adjective pallial) is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of ...

close to the animal's head. Specifically, streptoneury is the crossing of the cerebrovisceral

connectives In logic, a logical connective (also called a logical operator, sentential connective, or sentential operator) is a logical constant. They can be used to connect logical formulas. For instance in the syntax of propositional logic, the binary co ...

caused by this torsion. In a streptoneurous animal, the right visceral nerve becomes a supraintestinal nerve (i.e., moving up from its former position along the intestines on the right side) and the left visceral nerve becomes a subintestinal nerve (i.e., moving down below the intestines and shifting to the right). The extant gastropod molluscs can be described as streptoneurous. However, some lineages, especially the

opisthobranch Opisthobranchs () is now an informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping. Euopisth ...

s, have reversed this twist secondarily. The term for this event is ''detorsion'', and the condition in a detorted animal— a state in which these nerves no longer cross— is called

euthyneury Euthyneury is a plesiomorphic condition present in some gastropods which is a result of two evolutionary events. The first event, which was experienced by the ancestors of all extant gastropods, is known as torsion. Torsion describes the event in ...

References

Gastropod anatomy {{gastropod-stub