2.2.1 skeletons  (Page 3/4)

• Most terrestrial Arthropods have a tracheal system of breathing whereby air is forced in and out of a system of tubes by means of contraction and relaxationof muscles attached to the abdominal exoskeleton.
• Most insects have one or two pairs of wings which are formed from outgrowths in the thoracic region of the exoskeleton
• The jointed appendages have sensory hairs and can be modified and adapted for sensory functions (antennae), food handling(mouthparts), swift and efficientwalking legs and swimming appendages.

The exoskeleton because of its restriction on the size of terrestrial Arthropods was a major factor in future prominence of the vertebrates.

Developmental progression in the vertebrate skeleton and associated organs.

The two major requirements for survival on land are the development of a suitable support system and an air breathing mechanism.

Fish

• Bony fish and cartilaginous fish (sharks, rays etc) , apart from the skull have a skeleton consisting of a vertebral column to which bands of muscle are attached on either side.
• When swimming a wave of contraction is produced on one side followed by another wave on the opposite side.
• These alternate waves of contraction gives rise to S-shaped side to side movement of the body which together with the sweeping movement of the tail drive the fish forward. The fins keep stability and play a role in steering.. This method oflocomotion depends on the resistance of the surrounding water and would not work on land.
• A prehistoric group of fish had lobed fins . These fish lived in swamps or ponds which often dried up.
• It is thought that they used their fins to drag themselves from a dried up pond to a wet one. Fossil forms seem to indicate that these fins contained bones .
• They also had primitive “lungs” which enabled them to inhale small amounts of air.
• Three existing fish which have similar features are the coelacanth from deep seas, the mudskipper which occurs in mangrove swamps and uses its front lobedfins to climb trees, and the lungfish which live in stagnant water surfacing to gulp air. None of these developed into Amphibia which arose from one or more ofthe extinct forms.

Amphibia

• Amphibia: have partially adapted to land conditions.
• Firstly they have developed lung breathing in adults while tadpoles use gills.
• Secondly they have developed limbs for locomotion on land..
• One group salamanders and newts (not found in Africa) resemble small lizards. Their legs are mostly at right angles to the body and are small . Like fish their bodies bend from side to side when they walk.

Frogs have adapted their skeletons to land conditions in the following ways:

• skull is much lighter and more flattened to allow for mobility on land.
• the flexible vertebral column of fish has been shortened and acts as a r igid frame for transmitting force to the body .
• There has been an extreme shortening of the body (9 vertebrae and there is no tail), as this would be a hindrance when leaping and landing
• there is a pectoral girdle which serves as support for the forelimbs which are used mainly to absorb weight during landing after a jump.
• the pelvic girdle is elongated and bear the hind limbs . When at rest the thigh, calf and foot of a frog are each about the same length. As the frog jumps each part of the leg straightens in turn whichresults in great leaping power The webbing between the toes increase the thrust in the air as well as when swimming in water.
• the frog’s leap is not only used to move from one point to another but is also a very effective way of escaping from a predator.