Calcareous deposits in the renal sac of a molgulid tunicate.

Weddellite (calcium oxalate dihydrate) and calcite (anhydrous calcium carbonate phase) are components of concretions in the renal sac of the ascidian tunicate Molgula manhattensis. The presence of weddellite along with urate in the concretions suggests a resemblance to human kidney stones, although, unlike the latter, the concretions in Molgula do not seem to be pathologic deposits.