A sclerite (
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
, ', meaning "
hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly to the hardened parts of
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 ...
exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...
s and the internal
spicules of invertebrates such as certain
sponges
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
and
soft corals
Alcyonacea, or soft corals, are an order of corals. In addition to the fleshy soft corals, the order Alcyonacea now contains all species previously known as "gorgonian corals", that produce a more or less hard skeleton, though quite different ...
. In
paleontology
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
, a scleritome is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.
Sclerites in combination
Sclerites may occur practically isolated in an organism, such as the
sting
Sting may refer to:
* Stinger or sting, a structure of an animal to inject venom, or the injury produced by a stinger
* Irritating hairs or prickles of a stinging plant, or the plant itself
Fictional characters and entities
* Sting (Middle-earth ...
of a
cone shell. Also, they can be more or less scattered, such as tufts of defensive sharp, mineralised bristles as in many marine
Polychaetes. Or, they can occur as structured, but unconnected or loosely connected arrays, such as the mineral "teeth" in the
radula
The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...
of many
Mollusca
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 es ...
, the
valves of
Chiton
Chitons () are marine molluscs of varying size in the class Polyplacophora (), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized.
They are also sometimes known as gumboots or sea cradles or coat-of-mail she ...
s, the beak of
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
, or the articulated exoskeletons of Arthropoda.
When sclerites are organised into an unarticulated structure, that structure may be referred to as a scleritome, a term largely used in
paleontology
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
.
Arthropods
In
Arthropoda
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 chitin ...
the hardening that produces sclerites is accomplished either by the cross-linking of protein chains in the
exocuticle, a process called
sclerotization, or by incorporation of minerals such as calcium carbonate into regions of the exoskeleton, or both. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by less-sclerotized, membranous regions or sutures.
Dorsal sclerites of a body segment, often plate-like, are known as ''
tergites''. Similarly the
ventral sclerites of a body segment are referred to as ''
sternites''. Separate sclerites on the lateral aspects of body segments, the
pleura
The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls.
The inner pleura, called the visceral pleura, covers the surface of each lung and dips b ...
, are called ''pleurites''.
Invertebrates other than arthropods
Wide ranges of sclerites of various kinds occur in various invertebrate
phyla Phyla, the plural of ''phylum'', may refer to:
* Phylum, a biological taxon between Kingdom and Class
* by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another
Phy ...
, including
Polychaeta
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ...
and
Mollusca
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 es ...
. Two
taxa
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
that routinely have the term applied however, are the
soft corals
Alcyonacea, or soft corals, are an order of corals. In addition to the fleshy soft corals, the order Alcyonacea now contains all species previously known as "gorgonian corals", that produce a more or less hard skeleton, though quite different ...
and the
Porifera
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
. In both those groups certain of their structures contain mineralised spicules of
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 ...
or
calcium carbonate that are of importance structurally and in defense.
Many other invertebrates grow a few hard parts, largely mineralised, as
statoliths and similar structures, but those are not generally referred to as sclerites.
Clamps, the main attachment structure of the parasitic
Polyopisthocotylean
monogenea
Monogeneans are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female repr ...
n flatworms, are composed of various sclerites and associated musculature, located on a posterior organ called the
haptor. Clamps are specialized structures attached to the host fish, generally to its gill.
Prehistory
The fossilized scleritome of ''''">thumb
The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb ...
A scleritome is a skeleton made of isolated elements, like the scales of organisms such as the halwaxiids, the teeth in a radula,
spicules
Spicules are any of various small needle-like anatomical structures occurring in organisms
Spicule may also refer to:
*Spicule (sponge), small skeletal elements of sea sponges
*Spicule (nematode), reproductive structures found in male nematodes ( ...
in a sponge skeleton, or the elements in a
conodont
Conodonts ( Greek ''kōnos'', " cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, whi ...
apparatus.
The term was coined by the palaeontologist
Stefan Bengtson.
Although sclerites are of considerable importance in the study of extant animals, in palaeontology they are of far greater relative importance because they often are the only parts of an animal that fossilize at all, let alone well or clearly. Many extinct groups are known only from sclerites, leaving moot the question of what their gross anatomy might have looked like.
An example of the use of the term in paleontology is to describe hollow calcium carbonate,
calcium sulfate
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...
or
calcium phosphate plates grown as body armor by a number of Early
Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
animals. Unlike
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...
spicules, Early Cambrian sclerites appear to be external armor rather than internal structural elements. Sclerites are found on a curious collection of early animals including a common spongelike animal called ''
Chancelloria''; an armored slug-like form ''
Wiwaxia''; an armored worm with a pair of
brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
-like shells ''
Halkieria''; and another armored worm ''
Microdictyon'' that is generally considered to be a
lobopod/
onychophore.
It has been suggested that the sclerites of the Cambrian ''Wiwaxia'' are homologous with the bristles of
annelid worms.
At least one modern
gastropod
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
mollusc living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.
References
{{reflist
Arthropod anatomy
Sponge anatomy
Mollusc anatomy