The term hypoathroid (Ancient Greek ''hypo-'', "under" + ''-athroid'', "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 hypoathroid state, the pleural ganglia of the "chest" and the pedal ganglia of the "feet" lie close to each other more or less underneath the gut, and they communicate with the cerebral ganglia via long connectives. It is a condition that is characteristic of 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 ...

, and represents one end of a three-part spectrum of such arrangements, the other two being the dystenoid system in which the pleural and cerebral ganglia are closer together but still distinctly separate, and the epiathroid condition in which the pleural, pedal, and cerebral ganglia all lie close together (characteristic, for example, 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 ...

. The centralization of the nervous system generally is considered evidence of an evolutionary advancement among molluscs, while the more diffuse condition is viewed as a sign of evolutionary proximity to the hypothetical "archimollusc" ancestor of all molluscs.

References

{{reflist Mollusc anatomy