Locomotion and Skeleton in Vertebrates

Swimming in Fishes

For swimming the fishes show the following adaptations. (1) The body is streamlined to reduce friction and to swim through the water easily. (2) Both bony fish and cartilaginous fishes have well developed fins which help them to move in water. (3) The dermal denticles (small tooth like structure) of cartilaginous fish and the scale of bony fish are kept moist by slimy exudation from mucus or oil glands and this also considerably reduces friction between fish and water. (4) The dorsal and anal fins prevent the body from rolling sideways. The pectoral and pelvic fins are used mostly for steering and balancing and help the fish to change its direction. They also control the pitch of the fish i.e. its upward or downward movement.  (5) The back bone is flexible and generally the vertebrae are not interlocked. On either side of the backbone are several blocks of muscles known as myotomes. By successive contractions and relaxation of each myotomes the fish produces undulating movements and move forward. (6) The lateral movements of the caudal fin contribute greatly to forward movement.

(7) The swim bladder is a long narrow thin walled air sac found in the body cavity of bony fishes only. It gives buoyancy to the fish so that it does not sink even when it is not moving. The main function of the swim bladder is to help the fish to swim upwards or downwards.

Fins of Ray Fish

Swimming in Fish

Locomotion in Amphibians

When the amphibians evolved from lobed finned fishes they faced the problem of gravity. At an early stage of amphibian evolution, vertebrae became more complex, linked together by interlocking articulation. Thus the backbone became strong, and flexible. Girdles evolved, that supported the weight of the body. One group of primitive amphibians wriggle along their belly on the ground with the help of segmented arranged muscles as it 'swim on land; with legs hardly touching the ground when moving deliberately. Only a few raise up their body on the legs which then propel them along as movable levers.

In the anurans, e.g. frogs, toads, have peculiar swimming and jumping methods of locomotion, by means of extensor thrusts of both kind of limbs acting together.

Locomotion in Reptiles

In the early amphibians the legs sprawled out from the body, so that the animal probably dragged itself over the ground. This type of locomotion also characterized in the first reptiles. Most of the muscular energy would have been spent just in holding up the weight of trunk, if it did not rest on the ground. In the early stage of reptilian evolution, however, there were trends towards bringing of the feet in beneath the body and raising the body clear of the ground, and the legs acting as props. This arrangement of legs to body led efficiently to walking and running type of locomotion in reptiles and mammals.

The skeleton of reptiles is highly ossified to provide greater support. Reptiles have cervical vertebrae. The atlas and axis provide greater freedom of movement for head. The ribs of reptiles are highly modified. The ribs of snakes have muscular connections to large belly scales to aid locomotion. The prehistoric reptiles walked on hind legs, i.e. they were bipedal. They had a narrow pelvis and a tail for balance. Bipedal locomotion freed the front appendages, which became adapted for capturing prey or flight in some animals.

Locomotion in Aves

The fore-limbs are modified into wings, with feathers for flight and very strong pectoral muscles which pull the wings up and down. The hindlimbs are adapted for walking, perching or swimming. The bone are light in weight containing air cavities. The sternum is broad, usually with a longitudinal ventral keel for the attachment of flight muscles. The body is covered with feathers and provides a large surface area to keep the birds in air. Feathers also keep the body warm, so that enough energy can be produced for flying.

Birds have a boat shaped body (streamlined), which offers minimum resistance to air during flight. All contour (quill) feathers lie flat and are directed backwards for the same purpose.

Mechanism of Flight: Two distinct types of flight may be considered (a) gliding (b) flapping.

(a) Gliding: This simplest type of flight is used for landing. In gliding the wings are outspread as aerofoils, (i.e., like airplane wing). Air flowing over the curved surface moves faster than it does over the lower surface. This effectively produce a high pressure on the lower surface and low pressure on the upper surface and so provides a lifting force, the amount of lift depends on the angle at which the wing is held relative to the air stream. This is the passive form of flight.

Locomotion in Aves

(b) Flapping: In flapping flight the large pectoral muscle contracts, pulling the forelimb down. The resistance of the air to the wing produces an upward reaction on the wing. This force is transmitted through the coracoid bones, to the sternum and so acts through the birds centre of gravity, lifting as a whole. In the down stroke the air is thrust backward and the bird moves forward. This is the active form of flight.

Locomotion in Mammals

In mammals the limb bones have undergone modification to form three forms of locomotion.

Mammals Feet

 

Plantigrade: Some mammals walk on their soles with palm and sole, wrist and ankle digits of hand and foot all tending to rest more or less on the ground. This is primitive form of locomotion. Examples: monkey, man, apes and bear.

Digitigrade: Some mammals tend to rest their digits only on the ground with wrist and ankle, palm and sole elevated. They run fast than the plantigrade. They also walk more silently and more agile. In these mammals the first digit is usually reduced or completely lost. Examples: rabbits, rodents and many carnivores.

Unguligrade: These mammals walk on the tips of their toes which are hoofed. The weight of the body Is supported on the tips of the toes. In this case the palm and the sole, the ankle and the digits of the hand and foot are elevated with only the tips of the digits resting on the ground. Examples: Horse, goat, deer etc.

Ways of Locomotion

Bipedal locomotion: They walk, run or stand on their hind limbs. Such a type of locomotion commonly releases the forelimb for the feeding and other activities. Examples: Man and apes.

Gliding: Many animals that climb trees are too small to brachiate, consequently they jump from branch to branch. They usually flatten their body as they jump, thus making of themselves a sort of primitive gliders. In flying squirrel this trend has reached its climax with the formation of a large gliding membrane between front and the hind legs.

Example: Flying squirrels.

Brachiating: Animals move through the trees by using its grasping hands and long arms e.g. apes, monkeys.