Testaceans

Testaceans is a collective term used to describe a diverse group of marine and freshwater invertebrates that possess a shell or "test." This shell is typically made of calcium carbonate and serves as a protective outer covering. The term is not a strict taxonomic classification but rather a descriptive one encompassing a wide range of phyla, including mollusks, brachiopods, and foraminifera, among others.

Within mollusks, common testaceans include snails, clams, oysters, and mussels, all of which are characterized by their varied shell shapes and structures. Brachiopods, often mistaken for bivalve mollusks, are a separate phylum of marine animals that also have two shells. Foraminifera are single-celled organisms, primarily marine, that secrete shells of various forms, often intricate and microscopic.

The function of the test is primarily protection from predators and environmental stressors such as desiccation or extreme temperatures. The shell can also provide a point of attachment for muscles or serve as ballast. The diversity in shell morphology reflects the wide array of ecological niches occupied by these organisms, from shallow coastal waters to the deep sea. Fossilized tests are abundant in the geological record, providing valuable insights into past environments and evolutionary history.