Misplaced Pages

#702297

64-628: A skeleton is the structural frame that supports the body of most animals . There are several types of skeletons, including the exoskeleton , which is a rigid outer shell that holds up an organism's shape; the endoskeleton , a rigid internal frame to which the organs and soft tissues attach; and the hydroskeleton , a flexible internal structure supported by the hydrostatic pressure of body fluids . Vertebrates are animals with an endoskeleton centered around an axial vertebral column , and their skeletons are typically composed of bones and cartilages . Invertebrates are other animals that lack

128-444: A byssus ; other groups of bivalves (such as oysters , thorny oysters , jewel boxes , kitten's paws , jingle shells , etc.) cement their lower valve to a hard substrate (using shell material as cement) and this fixes them permanently in place. In many species of cemented bivalves (for example the jewel boxes), the lower valve is more deeply cupped than the upper valve, which tends to be rather flat. In some groups of cemented bivalves

192-411: A direction between the other two poles. Those ridges at the edge of the shell also play a major part in distributing force and allowing for a stronger shell. Serrate margins describe the ripple pattern around the edge of the bivalve shell. Compression tests have revealed that the presence of those ridges allows for more resistance to fracture than those with polished edges. It is difficult to summarize

256-462: A distinctive "comb" or ctinoleum within the byssal notch on the right valve. If a valve has neither notch nor comb nor sinus, and the auricles are of the same size, it is likely to be a left valve. In those animals whose valves have an umbo that seems to "point", that point is most often towards the anterior part of the valve (though there are some exceptions to this rule). Also, in those bivalves with two adductor muscle scars of different sizes,

320-413: A mixture of proteins , polysaccharides , and water. For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton. Rigid skeletons are not capable of movement when stressed, creating a strong support system most common in terrestrial animals . Such

384-456: A pocket-like space in the mantle . This feature of the internal anatomy of a bivalve is clearly indicated on the interior of the shell surface as a pallial sinus , an indentation in the pallial line. In addition, the water flows through incurrent siphon ventrally and exit out of the body through excurrent dorsally to the body. The valves of the shell are made of either calcite (as with, e.g. oysters) or both calcite and aragonite , usually with

448-401: A single tail fin as in the pinnipeds (seals). In the whale, the cervical vertebrae are typically fused, an adaptation trading flexibility for stability during swimming. The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as

512-818: A skeleton type used by animals that live in water are more for protection (such as barnacle and snail shells) or for fast-moving animals that require additional support of musculature needed for swimming through water. Rigid skeletons are formed from materials including chitin (in arthropods), calcium compounds such as calcium carbonate (in stony corals and mollusks ) and silicate (for diatoms and radiolarians ). Hydrostatic skeletons are flexible cavities within an animal that provide structure through fluid pressure, occurring in some types of soft-bodied organisms , including jellyfish, flatworms , nematodes , and earthworms. The walls of these cavities are made of muscle and connective tissue. In addition to providing structure for an animal's body, hydrostatic skeletons transmit

576-484: A smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true jaw , instead having evolved a beak , which is far more lightweight. The beaks of many baby birds have a projection called an egg tooth , which facilitates their exit from the amniotic egg. The skeleton, which forms the support structure inside the fish is either made of cartilage as in the Chondrichthyes , or bones as in

640-534: A vertebral column, and their skeletons vary, including hard-shelled exoskeleton ( arthropods and most molluscs ), plated internal shells (e.g. cuttlebones in some cephalopods ) or rods (e.g. ossicles in echinoderms ), hydrostatically supported body cavities (most), and spicules ( sponges ). Cartilage is a rigid connective tissue that is found in the skeletal systems of vertebrates and invertebrates. The term skeleton comes from Ancient Greek σκελετός ( skeletós )  'dried up'. Sceleton

704-491: Is a type of dense connective tissue , a type of mineralized tissue that gives rigidity and a honeycomb-like three-dimensional internal structure. Bones also produce red and white blood cells and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include bone marrow , endosteum and periosteum , nerves , blood vessels and cartilage. During embryonic development , bones are developed individually from skeletogenic cells in

SECTION 10

#1732845075703

768-406: Is added to, and increases in size, in two ways—by increments added to the open edge of the shell, and by a gradual thickening throughout the animal's life. The two shell valves are held together at the animal's dorsum by the ligament , which is composed of the tensilium and resilium. In life the ligament opens the shell (like a bent eraser in a door hinge), and the adductor muscle or muscles close

832-581: Is an archaic form of the word. Skeletons can be defined by several attributes. Solid skeletons consist of hard substances, such as bone , cartilage , or cuticle . These can be further divided by location; internal skeletons are endoskeletons, and external skeletons are exoskeletons. Skeletons may also be defined by rigidity, where pliant skeletons are more elastic than rigid skeletons. Fluid or hydrostatic skeletons do not have hard structures like solid skeletons, instead functioning via pressurized fluids. Hydrostatic skeletons are always internal. An exoskeleton

896-462: Is an external skeleton that covers the body of an animal, serving as armor to protect an animal from predators. Arthropods have exoskeletons that encase their bodies, and have to undergo periodic moulting or ecdysis as the animals grow. The shells of molluscs are another form of exoskeleton. Exoskeletons provide surfaces for the attachment of muscles, and specialized appendanges of the exoskeleton can assist with movement and defense. In arthropods,

960-412: Is cartilage which in mammals is found mainly in the joint areas. In other animals, such as the cartilaginous fishes , which include the sharks , the skeleton is composed entirely of cartilage . The segmental pattern of the skeleton is present in all vertebrates, with basic units being repeated, such as in the vertebral column and the ribcage. Bones are rigid organs providing structural support for

1024-561: Is measured in the normal direction, to past 350 MPa when calculated from compressive tests. While each type of bivalve varies greatly in their final measured strengths and properties, they share the same trends in how microstructure and even nanostructure affect the trends in those properties. A few groups of bivalves are active swimmers like the scallops ; many bivalves live buried in soft sediments (are infaunal ) and can actively move around using their muscular foot; some bivalves such as blue mussels attach themselves to hard substrates using

1088-408: Is said to be equivalved ; if the valves vary from each other in size or shape, inequivalved . If symmetrical front-to-back, the valves are said to be equilateral , and are otherwise considered inequilateral . The bivalve shell not only serves as protection from predators and physical damage, but also for adductor muscle attachment, which can allow the mollusc to "swim" short distances by flapping

1152-438: Is the enveloping exoskeleton or shell of a bivalve mollusc , composed of two hinged halves or valves . The two half-shells, called the "right valve" and "left valve", are joined by a ligament and usually articulate with one another using structures known as "teeth" which are situated along the hinge line . In many bivalve shells, the two valves are symmetrical along the hinge line — when truly symmetrical, such an animal

1216-416: Is used in some invertebrates, for instance in the hinge of bivalve shells or the mesoglea of cnidarians such as jellyfish . Pliant skeletons are beneficial because only muscle contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape. Cartilage is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from

1280-690: Is wider and shallower than the male pelvis. Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. The angle between the pubic bones is known to be sharper in males, which results in a more circular, narrower, and near heart-shaped pelvis. Invertebrates are defined by a lack of vertebral column, and they do not have bone skeletons. Arthropods have exoskeletons and echinoderms have endoskeletons. Some soft-bodied organisms, such as jellyfish and earthworms , have hydrostatic skeletons. The skeletons of arthropods , including insects , crustaceans , and arachnids , are cuticle exoskeletons. They are composed of chitin secreted by

1344-496: The Osteichthyes . The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. They are supported only by the muscles. The main external features of the fish, the fins , are composed of either bony or soft spines called rays which, with the exception of the caudal fin (tail fin), have no direct connection with

SECTION 20

#1732845075703

1408-496: The Pacific Ocean , has a shell that is massive in both size and weight. Syrinx aruanus is a species of sea snail with a very large shell. Endoskeletons are the internal support structure of an animal, composed of mineralized tissues , such as the bone skeletons found in most vertebrates. Endoskeletons are highly specialized and vary significantly between animals. They vary in complexity from functioning purely for support (as in

1472-404: The epidermis . The cuticle covers the animal's body and lines several internal organs, including parts of the digestive system. Arthropods molt as they grow through a process of ecdysis , developing a new exoskeleton, digesting part of the previous skeleton, and leaving the remainder behind. An arthropod's skeleton serves many functions, working as an integument to provide a barrier and support

1536-474: The ossicles that articulate only with each other. The hyoid bone , which is located in the neck and serves as the point of attachment for the tongue , does not articulate with any other bones in the body, being supported by muscles and ligaments. There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the hip bones on each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether

1600-476: The posterior scar will be the larger of the two and will be visible on both valves— this condition is referred to as being anisomyarian ; if the scars are of equal size, this is termed isomyarian ; if the valve has only one muscle scar, this is termed monomyarian . Furthermore, in those animals with a distinct external ligament, the ligament is usually to the posterior side of the umbo of both valves. Using one or more of these guidelines should strongly suggest

1664-451: The 65 that is typical in lizards. The skeletons of birds are adapted for flight . The bones in bird skeletons are hollow and lightweight to reduce the metabolic cost of flight. Several attributes of the shape and structure of the bones are optimized to endure the physical stress associated with flight, including a round and thin humeral shaft and the fusion of skeletal elements into single ossifications . Because of this, birds usually have

1728-541: The animal ages. Sea urchins have as many as ten variants of stereome structure. Among extant animals, such skeletons are unique to echinoderms, though similar skeletons were used by some Paleozoic animals. The skeletons of echinoderms are mesodermal , as they are mostly encased by soft tissue. Plates of the skeleton may be interlocked or connected through muscles and ligaments. Skeletal elements in echinoderms are highly specialized and take many forms, though they usually retain some form of symmetry. The spines of sea urchins are

1792-432: The animal has these structures) and the posterior or back of the shell is where the siphon is located (again, if present— the scallops , for example, do not have siphons). Without being able to view these organs, however, determining anterior and posterior can be rather more difficult. In those animals with a siphon, the pallial sinus of the siphon, which will be present on both the left and right valves, will point towards

1856-423: The animal's posterior — such valves are called sinopalliate . Shells without a pallial sinus are termed integripalliate — such animals (as mentioned, the scallops as well as some other groups) often have a byssal notch present on the anterior end of the right valve (only), and the anterior auricles or "wings" of both valves will be either larger than, or equal to, the posterior ones. Such valves may also have

1920-415: The animal's body. The skeletons of sea cucumbers are an exception, having a reduced size to assist in feeding and movement. Echinoderm skeletons are composed of stereom , made up of calcite with a monocrystal structure. They also have a significant magnesium content, forming up to 15% of the skeleton's composition. The stereome structure is porous, and the pores fill with connective stromal tissue as

1984-525: The anterior/ posterior orientation of any given bivalve shell, and therefore whether any particular shell belongs to the right side or the left. The age of bivalve molluscs can be estimated in several ways. The Noah's Ark clam Arca noae has been used to compare these methods: the annual growth rings on the exterior of the valves can be counted at one per year and give a satisfactory result, but sometimes spurts of growth occur which may create an extra ring and cause confusion. Early rings may get worn away near

Skeleton - Misplaced Pages Continue

2048-496: The aragonite forming an inner layer, as is the case with the Pteriida which have this layer in the form of nacre or mother of pearl. The outermost layer of the shell is known as the periostracum and is composed of a horny organic substance. This sometimes forms a yellowish or brownish "skin" on the outside of the shell. The periostracum may start to peel off of a shell when the shell is allowed to dry out for long periods. The shell

2112-463: The body, assistance in movement by opposing muscle contraction , and the forming of a protective wall around internal organs. Bones are primarily made of inorganic minerals, such as hydroxyapatite , while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones are osteoblasts , osteoclasts , or osteocytes . Bone tissue

2176-399: The body, providing appendages for movement and defense, and assisting in sensory perception. Some arthropods, such as crustaceans, absorb biominerals like calcium carbonate from the environment to strengthen the cuticle. The skeletons of echinoderms , such as starfish and sea urchins , are endoskeletons that consist of large, well-developed sclerite plates that adjoin or overlap to cover

2240-432: The brain, lungs , heart and spinal cord . The biggest bone in the body is the femur in the upper leg, and the smallest is the stapes bone in the middle ear . In an adult, the skeleton comprises around 13.1% of the total body weight, and half of this weight is water. Fused bones include those of the pelvis and the cranium . Not all bones are interconnected directly: There are three bones in each middle ear called

2304-448: The case of sponges ), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived from mesodermal tissue. Endoskeletons occur in chordates , echinoderms, all great apes (including humans), and sponges. Pliant skeletons are capable of movement; thus, when stress is applied to the skeletal structure, it deforms and then regains its original shape. This skeletal structure

2368-418: The coccyx or tail bone is counted as one or four separate bones, and does not count the variable wormian bones between skull sutures. Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in

2432-422: The ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone. Cartilage is also used in vertebrates to resist stress at points of articulation in

2496-471: The edges of lakes, rivers and streams. They are collected by professional and amateur conchologists and are sometimes harvested for commercial sale in the international shell trade or for use in glue, chalk, or varnish, occasionally to the detriment of the local ecology. The bivalve shell is composed of two calcareous valves. The mantle , a thin membrane surrounding the body, secretes the shell valves, ligament , and hinge teeth . The mantle lobes secrete

2560-473: The effect of the environment on the health of the bivalve. For example, one type of bivalve, Cerastoderma edule , was studied with scanning electron microscopy (SEM) and nanoindentation to determine if exposure to higher levels of carbon dioxide would affect the structure of the shell. Fortunately for the bivalves, there appeared to be no strong correlation between exposure to high carbon dioxide partial pressures and shell hardness. The study did further confirm

2624-406: The exoskeleton also assists with sensory perception . An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The southern giant clam , a species of extremely large saltwater clam in

Skeleton - Misplaced Pages Continue

2688-438: The forces of muscle contraction, allowing an animal to move by alternating contractions and expansions of muscles along the animal's length. The cytoskeleton ( cyto- meaning 'cell') is used to stabilize and preserve the form of the cells. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion using structures such as flagella , cilia and lamellipodia , and transport within cells such as

2752-472: The general belief in a correlation between the size of bivalve microstructures and their properties, namely larger microstructures produced poorer results. There are many factors that can affect the strength of bivalve shells. The outermost part of the shell has lower porosity , which results in a lower strength, while moving towards the innermost part of the shell increases strength. A general reader may believe that defects and non-uniformity would decrease

2816-437: The largest type of echinoderm skeletal structure. Some molluscs, such as conchs, scallops, and snails, have shells that serve as exoskeletons. They are produced by proteins and minerals secreted from the animal's mantle . The skeleton of sponges consists of microscopic calcareous or siliceous spicules . The demosponges include 90% of all species of sponges. Their "skeletons" are made of spicules consisting of fibers of

2880-400: The lower or cemented valve is the left valve, in others it is the right valve. The oldest point of a bivalve shell is called the beak , and the raised area around it is known as the umbo (plural umbones ). The hinge area is the dorsum or back of the shell. The lower, curved margin is the ventral side. The anterior or front of the shell is where the byssus and foot are located (if

2944-435: The male and female skeletons. The male skeleton, for example, is generally larger and heavier than the female skeleton. In the female skeleton, the bones of the skull are generally less angular. The female skeleton also has wider and shorter breastbone and slimmer wrists. There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. The female pelvis

3008-509: The members designed to carry the loads, and all other members are referred to as non-structural. A classification for the structural system of a high-rise was introduced in 1969 by Fazlur Khan and was extended to incorporate interior and exterior structures. The primary lateral load-resisting system defines if a structural system is an interior or exterior one. The following interior structures are possible: The following exterior structures are possible: Bivalve shell A bivalve shell

3072-467: The members of the structures under major design loads. However any two or more of the basic structural types described in the following may be combined in a single structure, such as a building or a bridge in order to meet the structure's functional requirements. The structural system of a high-rise building is designed to cope with vertical gravity loads as well as lateral loads caused by wind or seismic activity. The structural system consists only of

3136-561: The most common structures to study are sheet nacreous, crossed-lamellar, and complex crossed-lamellar. On every order and structural hierarchy in the lamellae, a common structure to find is twinning , which occurs on both the microscale and nanoscale. Nanotwinning occurs with incoherent twin boundaries and grow preferentially in the (110) and (1-10) crystallographic directions . Studying how these structures affect properties like Young's modulus , hardness , and toughness can help find mechanisms to improve modern materials, as well as study

3200-432: The movement of vesicles and organelles , and plays a role in cellular division. The cytoskeleton is not a skeleton in the sense that it provides the structural system for the body of an animal; rather, it serves a similar function at the cellular level. Vertebrate skeletons are endoskeletons, and the main skeletal component is bone. Bones compose a unique skeletal system for each type of animal. Another important component

3264-452: The outer edge of each valve, usually joining the anterior adductor muscle scar to the posterior adductor muscle scar. The adductor muscles are what allow the bivalve to close the shell tightly. In some bivalves the mantle edges fuse to form siphons , which take in and expel water during suspension feeding . Species which live buried in sediment usually have long siphons, and when the bivalve needs to close its shell, these siphons retract into

SECTION 50

#1732845075703

3328-407: The outside of a bivalve shell, a viewer may notice several ridges along it. This is a hint that shells of bivalves experience anisotropy . For example, when a type of bivalve, Tridacna gigas, was modelled and analyzed, it was found to be highly oriented in along a singular axis. This occurrence was evidenced by the stiffest Young's modulus occurring at one set of poles on the material, and weakest in

3392-656: The protein spongin , the mineral silica , or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges . Cartilage is a connective skeletal tissue composed of specialized cells called chondrocytes that in an extracellular matrix . This matrix is typically composed of Type II collagen fibers, proteoglycans , and water. There are many types of cartilage, including elastic cartilage , hyaline cartilage , fibrocartilage , and lipohyaline cartilage. Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by

3456-421: The pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly. [REDACTED] Media related to Skeletons at Wikimedia Commons Structural frame Commonly used structures can be classified into five major categories, depending on the type of primary stress that may arise in

3520-545: The shell (like a person pulling the door closed by the handle). When a bivalve dies, its adductor muscle(s) relax and the resilium pushes the valves open. The mechanical properties of bivalve shells and their relatedness to microstructure was first published in 1969 by Stephen Wainwright at Duke University. Following this, eight main categories of bivalve microsections were defined: simple prismatic, composite prismatic, sheet nacreous , lenticular, foliated , crossed- lamellar , complex crossed-lamellar, and homogenous. Some of

3584-456: The skeleton. Cartilage in vertebrates is usually encased in perichondrium tissue. Ligaments are elastic tissues that connect bones to other bones, and tendons are elastic tissues that connect muscles to bones. The skeletons of turtles have evolved to develop a shell from the ribcage, forming an exoskeleton. The skeletons of snakes and caecilians have significantly more vertebrae than other animals. Snakes often have over 300, compared to

3648-430: The spine. They are supported by the muscles which compose the main part of the trunk. Cartilaginous fish, such as sharks, rays, skates, and chimeras, have skeletons made entirely of cartilage. The lighter weight of cartilage allows these fish to expend less energy when swimming. To facilitate the movement of marine mammals in water, the hind legs were either lost altogether, as in the whales and manatees , or united in

3712-475: The strength and Young's modulus for bivalves as a whole because they vary greatly between different types of bivalves and their testing conditions. The Young’s modulus in bivalves can run from as low as 11.8 GPa in the normal direction for Pinna muricata , to 77 GPa in the perpendicular direction for Pinctada maxima . Dried samples read higher Young’s moduli values when compared to their wet counterparts and bending strength runs from 31 MPa when Saccostrea cucullata

3776-473: The strength of the bivalve shell, but that is not necessarily the case. The length scale of defects in bivalve shells runs from millimeters to less than a nanometer and can form 1D, 2D, and 3D defects . The randomness of defects can decrease porosity, which prevents cracking. Along a similar vein, waviness in the lamellar planes will increase toughness, and increases in interfacial area, where two surfaces come into contact, will promote strength. When looking at

3840-458: The total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together. These bones are organized into a longitudinal axis, the axial skeleton , to which the appendicular skeleton is attached. The human skeleton takes 20 years before it is fully developed, and the bones contain marrow , which produces blood cells. There exist several general differences between

3904-412: The umbones and the narrow rings near the margin may be difficult to interpret in fully grown individuals. Similar annual pallial line scars on the interior of the valves are more easily seen in dark colored shells, but these may be overgrown and obscured by further deposition of hard material. Another method is the examination of the growth lines and bands seen in acetate peel replicas taken in the region of

SECTION 60

#1732845075703

3968-409: The umbones. The most accurate but most time-consuming method is the microscopic examination of sections through the outer prismatic layer of the shell. Using more than one of these methods should increase the accuracy of the result. The hinge teeth (dentition) or lack of them is an important feature of bivalve shells. They are generally conservative within major groups, and have historically provided

4032-413: The valves, and the mantle crest creates the other parts. The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell. The position of this line is often quite clearly visible on the inside of each valve of a bivalve shell, as a shiny line, the pallial line , which runs along a small distance in from

4096-412: The valves. The shell is secreted by a soft part of the molluscan body known as the mantle and has several layers, typically made of calcium carbonate precipitated out into an organic matrix. Bivalves are very common in essentially all aquatic locales, including saltwater , brackish water and fresh water . The shells of dead bivalves commonly wash up on beaches (often as separate valves) and along

#702297