The term epiathroid (Ancient Greek ''epi-'', "above" + ''-athroid'', "gathered together") is used to describe the arrangement of

ganglia A ganglion 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 are both sympatheti ...

in the nervous system of molluscs. 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 A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

. It is a condition characteristic of the

Mesogastropoda Mesogastropoda was for many years a traditional taxonomic group of snails, an order. The order was composed mostly of sea snails, but it also included some land snails and freshwater snails, all of which were prosobranch gastropod mollusks. Thi ...

and

Neogastropoda Neogastropoda is an order of sea 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 Creta ...

, and is the obverse of the more-primitive

hypoathroid The term hypoathroid (Ancient Greek ''hypo-'', "under" + ''-athroid'', "gathered together") is used to describe the arrangement of ganglia in the nervous system of molluscs. In the hypoathroid state, the pleural ganglia of the "chest" and the peda ...

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 sea snails used in older classifications of gastropods, i.e. snails and slugs. Archeogastropoda are marine prosobranch gastropod mollusks, mainly herbivores, typically havi ...

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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