Calcareous is used as an adjectival term applied to anatomical structures which are made primarily of calcium carbonate, in animals such as gastropods, i.e., snails, specifically in relation to such structures as the operculum, the clausilium, and the love dart. The term also applies to the calcium carbonate tests of, often, more-or-less microscopic Foraminifera. Not all tests are calcareous; diatoms and radiolaria have siliceous tests.
The molluscs are calcareous organisms, as are the calcareous sponges (Calcarea), that have spicules which are made of calcium carbonate.
Additionally, reef-building corals, or Scleractinia, are calcareous organisms that form their rigid skeletal structure through the precipitation of aragonite (i.e., a polymorph of calcium carbonate).