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 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 clade, and is the inverse of the evolutionarily more recent epiathroid condition, characteristic of the Mesogastropoda and Neogastropoda, in which the pleural, pedal, and cerebral ganglia all lie close together. This centralization of the nervous system is considered evidence of an evolutionary advancement, and the more diffuse condition a sign of evolutionary similarity to the original hypothetical "archimollusc" ancestor of all molluscs.
- Chase, Ronald (2002). Behavior and Its Neural Control in Gastropod Molluscs. Oxford University. p. 22. ISBN 9780195354485.