The term epiathroid (Ancient Greek ', "above" + ', based on the Ancient Greek for "gathered together") is used to describe the arrangement of

ganglia A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...

in the nervous system of

molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

. In the epiathroid state, the pleural ganglia of the "chest" and the pedal ganglia of the "feet" lie close to the cerebral ganglia of the "head" forming a neural cluster which begins to approximate a

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

. It is a condition characteristic of the

Mesogastropoda Mesogastropoda was for many years a traditional Taxonomy (biology), taxonomic group of snails, an Order (biology), order. The order was composed mostly of sea snails, but it also included some land snails and freshwater snails, all of which were p ...

and

Neogastropoda Neogastropoda is an order of aquatic snails, both freshwater and marine gastropod molluscs. Description The available fossil record of Neogastropoda is relatively complete, and supports a widely accepted evolutionary scenario of an Early Cre ...

, and is the obverse of the more-primitive hypoathroid condition in which the pleural and pedal ganglia lie close together under the animal's gut and communicate with the cerebral ganglia via long connectives. The

Archaeogastropoda Archaeogastropoda (also known as Aspidobranchia) was a taxonomic order of snails used in older classifications of gastropods, i.e. snails and slugs. Archeogastropoda are primarily marine prosobranch gastropod mollusks, mainly herbivores, typica ...

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

is described as "hypoathroid", and is the clade closest to the original hypothetical ancestral mollusc (sometimes called an "archimollusc" or a H.A.M.). In between these two extremes lie those animals with a dystenoid nervous system in which the pleural and cerebral ganglia are closer than they are in the hypoathroid condition but still further apart than they are in the epiathroid one.

References

Mollusc anatomy Animal nervous system {{mollusc-stub