Test (biology)


In biology, a test is the hard shell of some spherical marine animals, notably sea urchins and microorganisms such as testate foraminiferans, radiolarians, and testate amoebae. The term is also applied to the covering of scale insects. The related Latin term testa is used for the hard seed coat of plant seeds.

Etymology

The anatomical term "test" derives from the Latin testa. It is distinct from the term "test" as in "examination", which derives from testis, related to the idea of testimony.

Structure

The test is a skeletal structure, made of hard material such as calcium carbonate, silica, chitin or composite materials. As such, it allows the protection of the internal organs and the attachment of soft flesh.

In sea urchins

The test of sea urchins is made of calcium carbonate, strengthened by a framework of calcite monocrystals, in a characteristic "stereomic" structure. These two ingredients provide sea urchins with a great solidity and a moderate weight, as well as the capacity to regenerate the mesh from the cuticle. According to a 2012 study, the skeletal structures of sea urchins consist of 92% of "bricks" of calcite monocrystals and 8% of a "mortar" of amorphous lime. This lime is constituted itself of 99.9% of calcium carbonate, with 0.1% structural proteins, which make sea urchins animals with an extremely mineralized skeleton.

In foraminiferans

The test of foraminiferan initially consists of gradually mineralized organic matter, but also sometimes exogenous agglomerated particles. It can be of many types, like agglutinated, microgranular, porcelain-like or hyalin. Foraminiferans develops by building new rooms in their test. These are arranged according to a geometry particular to each species: they can be rectilinear, curved, rolled up or cyclic, and every time uniserial or multiserial. These organizational types can also be mixed, or even more complex. Miliolids have a particular arrangement. The surface of the test can be smooth or textured.

In ascidians

In ascidians the sheath is sometimes called test as well, and is composed largely of a particular type of cellulose historically termed "tunicine". From 1845 until 1958, ascidians were believed to be the only animals that synthesised cellulose.

Other terms

On a strictly scientific point of view, the term "test" should be restricted to the hard shell protecting sea urchins and foraminiferans. For sessile echinoderms, the correct word is "theca". For diatomea, the term in use is "frustule", and for radiolarians it should be "capsule". The more common word "shell" is used for mollusks, arthropods and turtles.