A hypothetical ancestral mollusc or HAM showing its hypoathroid nervous system in which the pleural and pedal ganglia are separated from the cerebral ganglia by long connectives.
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.
[1]
References