In biology, a test is the hard shell
of some spherical marine animals, notably sea urchin
s and microorganisms such as testate foraminiferan
s, 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.
The anatomical term "test" derives from the Latin ''testa'' (which means a rounded bowl, amphora or bottle).
The test is a skeletal structure, made of hard material such as calcium carbonate
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 urchin
s 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 (conferring solidity and hardness) and 8% of a "mortar" of amorphous lime (allowing flexibility and lightness). 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 (which also explains their excellent conservation as fossils).
The test of foraminifera
, a group of single-celled organisms, is extremely evolutionarily diverse. Many different methods of constructing the test are present, from lacking a test in ''Reticulomyxa
'', proteinaceous tests in the "allogromiids
", agglomerated tests made from foreign particles in many groups including textulariids
, silica tests in silicoloculinids
, and aragonite or calcite tests in many forms including miliolids
. It can be of many types, including proteinaceous, agglutinated (exogenous agglomerate), porcelain-like (smooth calcite) or hyalin (lens). Foraminifera with multi-chambered tests are referred to as ''multilocular'' and develop by building new chambers in their test. These are arranged according to a geometry particular to each species: they can be rectilinear, curved, rolled up or cyclic, uniserial or multiserial. These organizational types can also be mixed, or even more complex. Miliolids have a particular arrangement of chambers known as "milioline". The surface of the test can be smooth or textured, and may be perforated with small holes.
s the sheath is sometimes called test as well, and is composed largely of a particular type of cellulose
historically termed "tunicine". From 1845 (when this was discovered by Schmidt
) until 1958 (when cellulose fibres were found in mammalian connective tissue), ascidians were believed to be the only animals that synthesised cellulose.
The Test of the Ascidian, Phallusia mammillata
Quarterly Journal of Microscopical Science, Vol. 102, part 1, pp. 107-117, 1961.
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 (like crinoid
s, but also many fossile groups such as cistoid
s or blastoid
s), the correct word is "theca
". For diatomea
, the term in use is "frustule
", and for radiolarian
s it should be "capsule
". The more common word "shell
" is used for mollusk
s and turtle
s (even if the latter ones belong to the order "Testudines
Sea urchin tests.jpg|Sea urchin tests (''Coelopleurus exquisitus'' on a ''Phyllacanthus imperialis'').
Sea urchin test structure (8340336761).jpg|Test of a purple sea urchin.
Echinocardium-fd.jpg|Test of an irregular sea urchin (''Echinocardium'').
3339e Croatie Pag.jpg|Foraminiferans' tests from the Adriatic Sea.
* Lorica (biology)