Movement in most animals is aided by muscles, which are attached to a skeleton. Movement of the body from one place to another is called locomotion. Some animals show no locomotion but move their bodies by swaying while fixed in one place. These animals are sessile or sedentary like the sponges living in the water.
Movement allows the animal to search for food, find mates or territory. They also move to escape from an enemy. The skeletal system provides support for the body, gives the animal its shape and protects the internal organs. It also provides attachment for the muscles, necessary for movement.
This type of movement is characteristic of a protozoan such as the Amoeba. In addition, individual cells such as macrophages use amoeboid movement to capture bacteria and engulf them. Amoeboid movement involves the streaming of endoplasm towards the ectoplasm, which is then forced to bulge at some forming the pseudopodia. Other protozoan organisms use different organs of locomotion. Euglena uses flagella and the Paramecium uses cilia
Hydrostatic Skeleton
Water forms the major component of most living things and body support for the cells, tissues and organs. In small organisms such the Amoebae, Hydra and earthworm water acts as a hydrostatic skeleton that makes the organism move.
Segmented soft-bodied animals such as the earthworm have fluid filled skeletons and two types of muscles: the circular and longitudinal muscles When the muscles contract the water is compressed as if it were in a rubber membrane and the hydrostatic pressure generated causes the earthworm to move.
Hydrostatic pressure due to aqueous humour in the eye keeps the eyeball in a proper shape in plants turgor is a form of hydrostatic pressure.
Exoskeleton
Exoskeleton is characteristic of arthropods. It is hard outer armour made of protein and Carbohydrate called chitin. It is tough and rigid but soft at the joints, which allows movement. The exoskeleton is a dead substance and therefore it cannot grow and this limits the growth and size of arthropods. Once it has been made growth will only take place after it has been shed.
Normally the larva will shed its skeleton several times before it becomes and adult insect and each time it undergoes moulting its soft body is exposed to danger from predators such as ants.
Despite imposing limitation on growth and size, the exoskeleton is not only light but also tough. It is quite suited to providing both protection and movement of the insect. Consequently, insects are the most successful group of organisms today.
Endoskeleton
The mammalian endoskeleton consists of two types of connective tissue: cartilage and bone. Cartilage is produced by cells called chondrocytes and is found at the ends of bones where they meet joints and in the embryo. The embryonic cartilage is later replaced by bone. Cartilage is also present between the bones (vertebrae) of the spinal column where it serves as a cushion or shock absorber.
The amount of cartilage in the skeleton depends on the species of vertebrate and the age of the animal. In lower vertebrates like the lamprey, the skeleton is entirely composed of cartilage. In mammals the cartilage is dominant in the embryo but is restricted to a few sites and at the end of bones in the adult. Such sites include the trachea, bronchi, external ear, nose and parts of the ribs.
Cartilage and bone tissues
Bone is composed of two kinds of protein that are produced by bone cells or osteocytes. The bone cells replace the chondrocytes after the embryo has developed for about two months. The bone is infiltrated by various minerals particularly calcium and phosphate salts which gives it its characteristic hardness.
The vertebrate skeleton
The vertebrate skeleton is divisible into axial skeleton and appendicular skeleton.
The axial skeleton
The axial skeleton consists of the head, neck and trunk. The head case is the cranium, which protects the brain; the vertebral column or spine protects the spinal cord, and the trunk (ribs and the sternum) protects the heart and the lungs. The cranium consists of numerous bones that are tightly joined so that there is no movement between individual bones.
The bones of the vertebral column are known as vertebrae. Each vertebra contains projections for the attachment of muscles and for articulation with other vertebrae at its anterior and posterior ends, as well as canals for the passage of blood vessels. Cartilage intervertebral discs join adjacent vertebrae. The cartilage is a kind of shock absorber that prevents adjacent vertebrae from rubbing against each other. Nerve fibres leave the spinal cord through soft spots within intervertebral discs.
Humans have a total of 28 vertebrate, made up of cervical or neck vertebrae (7), thoracic or chest vertebrae (12), lumbar or abdominal vertebrae (5), sacral or hip vertebrae (5) and sacral or tail vertebrae (4).
The appendicular skeleton
The appendicular skeleton is made up of the arms, legs, pectoral and pelvic bones. It has movable joints that allow free movement of the limbs.
Joints
Joints are found where bones joined or linked.
The joints are joined together by ligaments and have cartilage at their ends. A fluid called the synovial fluid lubricates the joints. There are two types of joints. One is the hinge joint found in the elbow, knee and fingers. This kind of a joint allows movement in one plane only such as upwards or downwards.
The second joint is the ball and socket joint found in the shoulder and the hip. This type of joint allows movement in all planes as well and some rotation of the limbs.
Adaptation to flight in birds
Birds have feathers, which are outgrowths of the epidermis like the scales of reptiles and hairs of mammals. There are four types of feathers: flight, contour, down and plume feathers. Flight feathers are used for flight. The contour feathers give the bird’s body its streamline shape and the down and plume feathers are mainly used for insulation.
In addition to the feathers birds have many other adaptations that are suited to flight. These are listed below.
1. Hollow spongy bones that make the bird light.
2. A streamlined body, covered with feathers, reduces air resistance.
3.Lungs containing air sacs.
4.Modification of the sternum (breastbone) into a keel for the attachment of powerful pectoral muscles that move the wings
5.Maintenance of a high metabolic rate and a relatively high body temperature averaging 40o C
6.Possession of very acute vision
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