• To move, muscles must work in concert with a skeleton.• The skeleton provides a rigid structure to which muscles are attached.• Muscles exert force only during contraction. Moving a body part back & forth requires two muscles attached to the same section of the skeleton.• These two muscles are known as an antagonistic pair, which functions cooperatively. The nervous system coordinates these muscles.
The main function of a skeleton• Support• Protection – In many animals, the skeleton protects soft tissue. – For example, the vertebrate skull protects the brain whilst the ribs of terrestrial vertebrates forms a cage around the heart, lungs and other internal organs.• Movement
Types of skeletons• There are three types of skeletons, namely: – A Hydrostatic skeleton – An Exoskeleton , and – An Endoskeleton
Hydrostatic skeletons• Consists of fluid held under pressure in a closed body compartment.• Examples of animals containing a hydrostatic skeleton: – Cnidarians – Flatworms – Nematodes – Annelids
Hydrostatic skeletons• Movement and form is controlled by using muscles to change the shape of the fluid filled compartment.• A Hydra, for example, elongates by closing its mouth by using contractile cells in its body wall. This results in the constriction of its central gastro-vascular cavity.• By decreasing the diameter of the cavity, the cavity is forced to become longer.
Hydrostatic skeletons• In planarians & other flatworms, the interstitial fluid is kept under pressure & functions as the main hydrostatic skeleton.• Planarian movement results mainly from muscles in the body wall exerting localized forces against the hydrostatic skeleton.
Hydrostatic skeletons • Nematodes hold fluid in their body cavity, which is a pseudocoelom. • Contractions of longitudinal muscles move the animal forward by undulations (wavelike) motions, of the body. • In earthworms and other annelids the coelomic fluid functions as a hydrostatic skeleton. The coelomatic cavity in many annelids is divided by the septa between the segments; allowing the animal to change the shape of each segment individually, using both circular and longitudinal muscles. • Annelids use their hydrostatic skeleton for peristalses (movement produced by rhythmic waves of muscle contractions passing from front to back).
Hydrostatic skeletons • Well suited for live in an aquatic environment. • Cushion internal organs from shocks, provide support for crawling and burrowing (in terrestrial animals). • Cannot support terrestrial activities in which an animal’s body is held off the ground, thus walking and running for example.
Exoskeletons• Hard, encasement deposited on an animal’s surface; for example, most molluscs (slugs and snails) are enclosed in a calcium carbonate shell secreted by the mantle (sheet like extension of the body wall).• As the animal grows, it enlarges its shell by adding to the outer edge.
Exoskeleton• Jointed exoskeleton arthropods have a cuticle, which is secreted by the epidermis. Muscles are attached to knobs and plates of the cuticle that extend into the interior of the body.• 30 – 50% of the cuticle consist of chitin, a polysaccharide similar to cellulose. Fibrils of chitin is embedded in a protein matrix, which forms a composite material that combines strength and flexibility.
Exoskeletons• For example, crustaceans (like lobsters) harden portions of their exoskeleton by adding calcium salts.• In leg joints, where the cuticle is thin and flexible, cross linking of proteins and inorganic salt decomposition occurs.• With each growth spurt, an arthropod must shed its exoskeleton to produce a larger one, thus they molt.
Endoskeletons • Consist of hard, supporting elements, such as bones which is buried within the soft tissue of animals. • Sponges are reinforced by hard, needle like structures of inorganic materials, or by softer fibres made of protein. • Echinoderms have an endoskeleton of hard plates called ossicles beneath their skin. • Ossicles are composed of magnesium carbonate, calcium carbonate crystals and are usually bound together by protein fibres.
Endoskeletons• Ossicles of sea urchins are tightly bound whilst ossicles of sea stars are more loosely linked. This allows the sea star to change the shape of its arms.• A chordate’s skeleton consits out of caritlage, bone or a combination of both.• Mammalian skeletons are built from more than 200 bones; some are fused together, others are connected at joints by ligaments that allow freedom of movement.
Ball-and-socket joints• These joints are present where: – The humerus contacts the shoulder girdle – The femur contacts the pelvic girdle• Enable us to rotate our arms and legs.• Also allows for us to move our arms and legs in several planes.
Hinge joints• Found between the humerus and the head of the ulna.• Restricts movement to a single plane.
Pivot joints• Allows us to rotate our forearm at the elbow.• Movement of head from side to side.
Locomotion• Requires that an animal expend energy to overcome two forces that tend to keep an organism stationary.• These forces are: – Friction, and – Gravity.• By exerting a force, energy is required which is produced during consuming cellular work.
Types of locomotion• Locomotion can be accomplished by means of three methods, namely: – Swimming, – Locomotion on land, and – Flying.
Swimming as locomotive tool• Overcoming gravity is easier in water, than it is for species who live on land. But, friction is a major problem for aquatic animals.• A common adaptation for fast swimmers is a sleek, fusiform (torpedo like) shape.
Swimming as locomotive tool – diverse adaptations• Insects and four-legged vertebrates use their legs as oars to push against the water.• Squids, scallops and some cnidarians are jet- propelled; they take in water and squirt it out in bursts in order to move.• Sharks and bony fish swim by moving their body and tail from side to side.• Whales and dolphins move by undulating their body and tail up and down.
Locomotion on land• Walking, running, hopping and crawling animals must be able to support themselves and move against gravity. Air poses relatively little resistance at moderate speeds.• When a land animal walks, runs, or hop, its leg muscles expend energy not only to propel the animal, but also to keep it from falling down. With each step the leg muscles must overcome inertia by accelerating a leg from a standing start.• Strong skeletal support and powerful muscles are important.
Locomotion on land - diverse adaptations• Kangaroos have large, powerful muscles in their hind legs which is suitable for locomotion by hopping.• As a kangaroo lands after each leap, tendons in its hind legs momentarily store energy. The farther the animal hops, the more energy the tendons store.• Analogous to energy in a compressed spring, the energy stored in the tendons is available for the next jump and reduces the total amount of energy the animal must expend to travel.
Locomotion on land - diverse adaptations• Legs of an insect, dog and human also retain some energy during walking and running, but in a smaller share.• Another important prerequisite for walking, running and hopping is to maintain balance.• A kangaroo’s large tail help balance its body during leaps and also forms a stable tripod with its hind legs when the animal sits and moves slowly.
Locomotion on land - diverse adaptations• Bipedal animals (such as humans and birds) keep part of at least one foot on the ground when walking.• When running: – All four feet (or both for bipeds) may be off the ground briefly, but at running speeds it is momentum more than foot contact that keeps the body upright.
Locomotion on land - diverse adaptations• When crawling: – Much of the body is in contact with the ground, a crawling animal must exert considerable effort to overcome friction. – Earthworms crawl by means of peristalsis. – Many snakes crawl by undulating their entire body from side to side; assisted by large, moveable scales on its underside. When the snake’s body pushes against the ground, its scales tilt forward and then push backward against the ground.
Flying as locomotive tool • For flying animals, gravity is a major problem since the animal’s wings must develop enough lift to overcome gravity’s downward force. • The animal’s wing shape is important. – All types of wings have airfoils (structures whose shape alters air currents in a way that helps animals stay aloft). – The body usually have a fusiform shape help reduce the drag in air as it does in water.
Flying as locomotive tool• Flying animals are relatively light.• Body masses range from less to a gram to about 20 kg (largest flying birds).
Flying as locomotive tool -adaptations• Many structural adaptations contribute to a lower body mass.• For example: – Birds have no urinary bladder or teeth, they have relatively large bones with air filled regions. – This lessen the bird’s weight.
Worksheets• Complete the following worksheets for homework: – Diagram of the Human Skeleton – Fill in the missing words – Crossword one – Crossword two
References• Campbell, N. A., & Reece, J. B. (2008). Biology (8th ed.). San Francisco, California, United States of America: Benjamin Cummings.• Google images• Skeletal System Cloze (n.d.). Available from: http://www.bogglesworld.esl.com/skeletalsystem.htm (Accessed 29 April 2012)• Skeletal System Crossword 1 (n.d.). Available from: http://www.bogglesworld.esl.com/skeletalsystem.htm (Accessed 29 April 2012)• Skeletal System Crossword 2 (n.d.) Available from: http://www.bogglesworld.esl.com/skeletalsystem.htm (Accessed 29 April 2012)• Skeletal System Diagram (n.d.). Available from: http://www.bogglesworld.esl.com/skeletalsystem.htm (Accessed 29 April 2012)• You tube