In biology, a test is the hard shell of some spherical marine animals and protists, notably

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) o ...

s and microorganisms such as testate foraminiferans, radiolarians, and

testate amoebae Testate amoebae (formerly thecamoebians, Testacea or Thecamoeba) are a polyphyletic group of unicellular amoeboid protists, which differ from naked amoebae in the presence of a test that partially encloses the cell, with an aperture from which th ...

. The term is also applied to the covering of

scale insects Scale insects are small insects of the order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient grouping than t ...

. 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'' (which means a rounded bowl, amphora or bottle).

Structure

The test is a skeletal structure, made of hard material such as calcium carbonate,

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

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

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).

In foraminiferans

The test of

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

, 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 The Allogromiida is an order of single-chambered, mostly organic-walled foraminiferans, including some that produce agglutinated tests (Lagynacea). Genetic studies indicate that some foraminiferans with agglutinated tests, previously included in ...

", 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 and rotaliids. 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.

In ascidians

In ascidians the sheath is sometimes called test as well, and is composed largely of a particular type of

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

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.Endean
The Test of the Ascidian, Phallusia mammillata
Quarterly Journal of Microscopical Science, Vol. 102, part 1, pp. 107-117, 1961.

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 (like crinoids, but also many fossile groups such as cistoids or

blastoid Blastoids (class Blastoidea) are an extinct type of stemmed echinoderm, often referred to as sea buds. They first appear, along with many other echinoderm classes, in the Ordovician period, and reached their greatest diversity in the Mississi ...

s), the correct word is " theca". For

diatomea A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

, the term in use is " frustule", and for radiolarians it should be " capsule". The more common word " shell" is used for

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s and

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

s (even if the latter ones belong to the order "

Testudines Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

"). Sea urchin tests.jpg, Sea urchin tests (''

Coelopleurus exquisitus The exquisite urchin (''Coelopleurus exquisitus'') is a sea urchin species found off the coast of the island of New Caledonia in the Pacific Ocean. It is an epifaunal deepwater species living at depths between and was only identified and name ...

'' on a '' Phyllacanthus imperialis''). Sea urchin test structure (8340336761).jpg, Test of a

purple sea urchin ''Strongylocentrotus purpuratus'', the purple sea urchin, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico, to British Columbia, Canada. This sea urchin species is deep purple in color, and lives in lower int ...

. Echinocardium-fd.jpg, Test of an irregular sea urchin ('' Echinocardium''). 3339e Croatie Pag.jpg, Foraminiferans' tests from the

Adriatic The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Sea) to the ...

Sea.