
Ossicles are small
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines.
In zoology
''Calcare ...
elements embedded in the
dermis
The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
of the body wall of
echinoderm
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s. They form part of the
endoskeleton
An endoskeleton (From Ancient Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is a structural frame (skeleton) — usually composed of mineralized tissue — on the inside of an animal, overlaid by soft ...
and provide rigidity and protection. They are found in different forms and arrangements in
sea urchin
Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...
s,
starfish
Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...
,
brittle star
Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...
s,
sea cucumber
Sea cucumbers are echinoderms from the class (biology), class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number be ...
s, and
crinoid
Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...
s. The ossicles and spines (which are specialised sharp ossicles) are the only parts of the animal likely to be fossilized after an echinoderm dies.
Formation
Ossicles are created intracellularly by specialised secretory cells known as
sclerocyte
Sclerocytes are specialised cells that secrete the mineralized structures in the body wall of some invertebrates.
In sponges they secrete calcareous or siliceous spicules which are found in the mesohyl layer of sponges. The sclerocytes produce ...
s in the
dermis
The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
of the body wall of echinoderms. Each ossicle is composed of microcrystals of
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
arranged in a three-dimensional lattice known as a
stereom. Under
polarized light
, or , is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. One example of a polarize ...
the ossicle behaves as if it were a single crystal because the axes of all the crystals are parallel. The space between the crystals is known as the
stroma and allows entry to sclerocytes for enlargement and repair. The honeycomb structure is light but tough and
collagen
Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
ous
ligament
A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...
s connect the ossicles together. The ossicles are embedded in a tough connective tissue which is also part of the endoskeleton. When an ossicle becomes redundant, specialised cells known as
phagocyte
Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek ', "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek ...
s are able to reabsorb the calcareous material.
[Ruppert et al, 2004. pp. 876–878] All the ossicles, even those that protrude from the body wall, are covered by a thin layer of
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...
but functionally they act more like an
exoskeleton
An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...
than an endoskeleton.
Types of ossicle

Ossicles have a variety of forms including flat plates, spines, rods and crosses, and specialised compound structures including
pedicellaria
A pedicellaria (: pedicellariae) is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms (phylum Echinodermata), particularly in sea stars (class Asteroidea) and sea urchins (class Echinoidea).
...
e and
paxillae.
Plates are tabular ossicles that fit neatly together in a
tessellated manner. They form the main skeletal covering for sea urchins and sea stars.
Spines are ossicles that project from the body wall and articulate with other ossicles through
ball and socket joint
The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
s mounted on
tubercle
In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal.
In plants
A tubercle is generally a wart-like projectio ...
s.
[Ruppert et al, 2004. pp. 897–898] They are formed from crystals of calcite and can be solid or hollow, long or short, thick or thin and sharp or blunt.
[ The spines serve a protective function and are also used for locomotion.][
]Pedicellaria
A pedicellaria (: pedicellariae) is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms (phylum Echinodermata), particularly in sea stars (class Asteroidea) and sea urchins (class Echinoidea).
...
e are compound ossicles that articulate with other ossicles and protrude from the aboral
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Thi ...
(upper) surface of some sea stars (and also the test
Test(s), testing, or TEST may refer to:
* Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities
Arts and entertainment
* ''Test'' (2013 film), an American film
* ''Test'' (2014 film) ...
of sea urchins). They usually have short fleshy stalks and either two or three moveable ossicles forming a set of pincer-like jaws. They may be scattered over the surface or may be grouped around spines. Their function is to pick off debris so as to keep the surface clean and to prevent larva
A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
e of other invertebrates from settling and growing there.
Paxillae are small pillar-shaped ossicles with flat tops sometimes found covering the aboral surface of sea stars such as ''Luidia
''Luidia'' is a genus of starfish in the family Luidiidae in which it is the only genus. Species of the family have a cosmopolitan distribution.
Characteristics
Members of the genus are characterised by having long arms with pointed tips frin ...
'', ''Astropecten
''Astropecten'' is a genus of sea stars of the family Astropectinidae.
Identification
These sea stars are similar one to each other and it can be difficult to determine with certainty the species only from a photograph. To have a certain de ...
'' and '' Goniaster'' that live underneath sediment. Their stalks emerge from the body wall and their tops, each fringed with short spines, and abut each other to form a protective external false skin. Beneath this is a water-filled cavity which contains the madreporite and delicate gill structures known as papillae.[
]
Arrangement
Sea urchins are covered with plates which are usually fused together to give a rigid test
Test(s), testing, or TEST may refer to:
* Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities
Arts and entertainment
* ''Test'' (2013 film), an American film
* ''Test'' (2014 film) ...
, but in the order
Order, ORDER or Orders may refer to:
* A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
...
Echinothurioida, the test is leathery because the plates are separate. The test is divided into five segments that extend from the apex to the mouth. Each contains two ambulacral
Ambulacral is a term typically used in the context of anatomical parts of the phylum Echinodermata or class Asteroidea and Edrioasteroidea. Echinoderms can have ambulacral parts that include ossicles, plates, spines, and suckers. For example, sea ...
rows of plates alternating with two interambulacral rows. The ambulacral plates are each pierced by a pair of pores through which the active tube feet
Tube or tubes may refer to:
* Tube (2003 film), ''Tube'' (2003 film), a 2003 Korean film
* "Tubes" (Peter Dale), performer on the Soccer AM#Tubes, Soccer AM television show
* Tube (band), a Japanese rock band
* Tube & Berger, the alias of dance/e ...
are connected to the water vascular system. Ossicles in the form of spines connect to tubercles on some of the plates. Sea urchins have several types of pedicellariae, some of which are toxic. A ring of specialised plates surround the aboral pole consisting of five genital plates, one of which is the madreporite, and five smaller ocular plates. Other large specialist plates surround the mouth in a set of jaws known as Aristotle's lantern.[Ruppert et al, 2004. pp. 897–898]
Sea stars have separate plates giving flexibility to the disc and arms. They are arranged into interambulacral and ambulacral regions and the arms have an ambulacral groove on the underside from which the tube feet project. Other ossicles that may be present include pedicellariae and paxillae. There is often a large row of marginal plates adjoining the ambulacral groove, sometimes bearing spines.[
Brittle stars do not have pedicellariae, and the plates that cover their surface are known as shields. On the arms these are in four rows with each segment having an aboral and oral shield and two lateral shields, usually with fringing spines. Other ossicles include spines, tubercles, small scales and vertebrae. The large central vertebrae in each arm segment provides the articulating element that joins it to the next.][Ruppert et al, 2004. p. 890]
Several types of small ossicles are found in the body wall of sea cucumbers. Baskets are cup-shaped and usually have four projections. Buttons are disc-shaped and pierced by four holes and may be smooth or knobbed. Perforated plates are sieve-like and often widely distributed and rods provide support for the tube feet and tentacle
In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s.[ In the order ]Apodida
Apodida is an order (biology), order of littoral to deep-sea, largely Fauna#Infauna, infaunal Holothuroidea, holothurians, sea cucumbers. This order comprises three families, 32 genus, genera and about 270 known species, called apodids, "without ...
, members of which lack tube feet, there are anchor-shaped ossicles attached to anchor plates. The flukes project from the body wall and provide traction.[Ruppert et al, 2004. p. 910]
Crinoids are supported by jointed stalks containing substantial compound ossicles. The crown has ossicles scattered throughout the connective tissue (crinoids have no distinct dermis). The arms contain columns of well-developed vertebrae-like ossicles. Each joint has limited movement but the whole arm can be coiled and uncoiled.[Ruppert et al, 2004. p. 917]
References
Bibliography
* {{cite book , title=Invertebrate Zoology, 7th edition , last1=Ruppert , first1=Edward E. , last2=Fox , first2=Richard, S. , last3=Barnes , first3=Robert D. , year=2004 , publisher=Cengage Learning , isbn=81-315-0104-3
Echinoderm anatomy