Skeleton is the bony framework of the body which provides support, shape and protection to the soft tissues and organs in animals. Without the skeleton, animals may not be able to move or carry out other life processes.

The skeleton of animals enable them to move from place to place. The skeleton, depending on the position of certain organism in the evolutionary trend, helps to determine the advancement and development of organisms for example, man is able to stand erect than other organisms due to its sophisticated skeletal build up.


There are three forms of skeletal materials found in animals. These materials are cuticle bones and cartilages.

(1) Cuticle: This is composed of a protein called chitin and a thin water-proof layer of wax. Chitin is a non-living substance; hence, animals with this type of skeletal material can only grow by moulting or ecdysis. In this process, an organism sheds off its old skeleton and put on a new one. In other words, for an organism to grow, it has to put off the old skin. Cuticle (Fig. 2.19) is an exoskeleton which is located externally on the body. Examples of organisms with this type of skeletal material (i.e. cuticle) are mainly the arthropods which are insects, crabs, scorpions and prawns.

(2) Bones: This is a tissue and a major component of the vertebral skeleton. It consists of living bone cells (osteocytes), protein fibres (collagen) and minerals, mainly calcium phosphate and calcium carbonate.

The mineral (non-living) component is made up of the mass of a bone. As a result, the bone is a stronger and more rigid tissue than the cartilage. A bone (Fig. 2.20) usually consists of a hard outer layer (shaft) and a spongy or hollow cavity filled with bone marrow. Examples of organisms which have bones are mainly vertebrates, which are bony fishes, toads, lizards, snakes, birds and mammals.

(3) Cartilages: This is a tissue found in the skeleton of complex vertebrates. It consists of living cells (chondroblasts), carbohydrates and protein fibres. It is a tough and flexible tissu that has great tensile strength. It acts as a shoc absorber, cushioning the effect of bones movin against bones during movement. Examples o organisms which possess cartilages an cartilaginous fishes like sharks, rays an mammals generally.


Types of Cartilages

In mammals, there are three types of cartilage:

(a) Hyaline Cartilage: This is found in (i) trachea and bronchi which keep them open (ii) surfaces of moveable joints (iii) the protruding part of the nose which supports it.

(b) a Fibro Cartilage: It is tougher than hyaline cartilage and it is found in the discs art between the small bones (vertebra) of the vertebral column.

(c) Elastic Cartilage: This is found in the external ear (pinnae) and epiglottis.

The vitamin and mineral element necessary for healthy bone development are:

(1) Vitamin -D/Calciferol, Vitamin C (form the cement of bone)

(2) Mineral elements – calcium/phosphorus/magnesium.

Types of Skeleton

There are three main types of skeleton. These are hydrostatic skeleton, exoskeleton and endoskeleton.

Hydrostatic (Fluid) Skeleton Hydrostatic Skeleton is the type of skeleton possessed by soft-bodied animals. They have luid pressure to provide support. Fluid is secreted to fill the spaces in the body. The fluid presses against the muscular body wall, causing the muscles to contract, exerting pressure against the fluid. This helps to maintain the shape and form of the animal. Examples of organisms with this type of skeleton is the earthworm and anemones.


Exoskeleton is the type of skeleton which is found outside or external part of the body of some animals. Most invertebrates do possess cuticle which is composed of chitin. Chitin is a non-living substance commonly found covering the outer part of the body of some animals. Such external skeletal tissue encloses, supports, gives shape, protects and also enable the animals to move.

Examples of organisms with exoskeleton are invertebrates like tapeworms, snails, insects, prawns, crabs, and crayfish. Organisms with this type of skeleton can only grow by a process called moulting or ecdysis. In this process, an organism sheds off its old skeleton, grow and later is covered with a new one.


Endoskeleton is the type of skeleton which is found inside the body of animals. Endoskeleton exists in bony or cartilagenous skeleton of fishes, toad, lizards, birds and mammals. Endoskeleton in vertebrates is made of cartilages and bones. Endoskeleton in mammals are the skull, vertebral column or back-bones, ribs and the bones of the fore and hind limbs.

Bones of Axial and Appendicular Skeleton The skeletal system or bones in mammals e.g and that of the rabbit are grouped into two major parts. These are the axial and appendicular skeleton.

(A) Axial skeleton: This is made up of the skull, vertebral column or backbone, the rib and sternum or breastbone.

(B) Appendicular skeleton: This is made up of the limb girdles (pectoral and pelvic girdles) and the limbs (fore limbs and hind limbs).

The Skull: The mammalian skull is mads up of several flat bones which are joined together by means of joints called sutures.

(i) The cranium or brain box which holds or contains the brain.

(ii) The facial skeleton which supports the nose, eyes and the muscles of the cheek.

(iii) The jaws which are made up of the upper jaw (maxilla) and lower jaw (mandible) which contain the teeth.

Functions of the Skull

(i) It protects the brain.

(ii) It gives shape to the head.

(iii) It protects vital organs in the head, e.g. eyes, nose and ears.

(iv) It bears the teeth which aids grinding of food.

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