Origin of the word
In an anatomical context, the word "test" is not linked to the empirical definition of "Test (assessment)" : the first one comes from the Latin "testa" (which means a rounded bowl, amphora or bottle), and the latter one from "testis", related to the idea of testimony.
The anatomical "test" is hence comprised in the same etymological field as "testicle", which also refers to the idea of a small, rounded bottle. It also gave the word for "head" in most roman languages (French tête, Italian testa...).
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 monocristals, 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 skelettal structures of sea urchins consist in "92% of bricks of calcite monocristals (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 % only of structure proteins, which make sea urchins animals with an extremely mineralized skeleton (which also explains their excellent conservation as fossiles).
The test of foraminiferan initially consists of gradually mineralized organic matter, but also sometimes exogenous agglomerated particles (in particular for species living in the sediment or in great depth). It can be of many types, like agglutinated (exogenous agglomerate), microgranular (with calcite grains), porcelain-like (smooth calcite) or hyalin (lens). 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 covered of streak, of ribs, of reticulate patterns, tubers, spines...
In ascidians the sheath is sometimes called test as well, and is composed largely of a pafticular 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.
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 crinoids, but also many fossile groups such as cistoids or blastoids), 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 (even if the latter ones belong to the order "Testudines").
Sea urchin test structure (8340336761).jpg
Test of a purple sea urchin.
Test of an irregular sea urchin (Echinocardium).
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- Endean, The Test of the Ascidian, Phallusia mammillata, Quarterly Journal of Microscopical Science, Vol. 102, part 1, pp. 107-117, 1961.