Molecular genetics, an important area within Biology Topics, focuses on the structure and function of genes.
What is the movement of animals on land?
It is necessary for the animals to be able to move from one place to another to find food and escape from their enemies. Many animals (including human beings) walk with their legs. Human beings walk by contracting and relaxing muscles in their legs in a co-ordinated manner. The action of flexor and extensor muscles in the legs bends and straightens the legs alternately and makes us walk (or move). This happens as follows : When we straighten the leg and push the ground backwards with our foot, the ground pushes our foot forward with an equal force.
This forward push of ground on our foot, makes us move ahead. In fact, every animal has to push something ‘backwards’ with its body part to be able to move ‘forward’. For example, a fish pushes on water backwards with its tail to move forward. We will first discuss the movement in a cockroach and a bird. Cockroach and birds are the animals which have legs to walk on ground and wings to fly in air.
Cockroach is an insect (see Figure). The body of a cockroach is covered with hard and tough outer covering called ‘exoskeleton’. The skeleton of cockroach is called exoskeleton because it is outside the body (‘exo’ means ‘outside’). The exoskeleton provides great protection to the internal organs of cockroach and also provides sites for the attachment of muscles. The outer skeleton of cockroach is made of different units joined together and that allows movement. The cockroach moves on ground as well as flies in air.
A cockroach has six legs (three legs on each side of its body) (see Figure). The cockroach moves (or walks) on the ground by using its legs. Each leg of cockroach consists of stiff, hollow tubes joined together. The legs of cockroach can be moved easily by the muscles. The muscles which move the legs of cockroach are attached to the inside of the exoskeleton. The flexor muscles bend the legs whereas extensor muscles straighten the legs and make the cockroach move (or walk).
A cockroach has two pairs of wings attached to its breast by flight muscles. The cockroach flies in air by moving its wings up and down rapidly with the help of flight muscles. When the wings of cockroach move down, they push on air downward and backward. The downward push on air lifts the cockroach up MgiuiellTZ A cockroach into air, and the backward push on air makes it move forward. After making the downstroke of wings, the cockroach raises its wings up again and then makes another downstroke. This keeps the cockroach flying.
Sparrow, crow, pigeon, parrot, eagle, hen, duck, swan, and peacock, are all birds. Birds walk on the ground and also fly in air. Some birds (like duck and swan) also swim in water. Birds walk on the ground by using their hind limbs (see Figure). Those birds which swim in water do so by pushing against water with their hind limbs which have webbed feet. The webbed feet help the bird in swimming in water.
The birds can fly because their bodies are adapted (or modified) for this purpose. The main adaptations which have been made in the bodies of birds by nature to enable them to fly are the following :
- Their forelimbs are modified to form wings for flying.
- Their flight feathers provide a large flat surface which is light but strong.
- Their bones are hollow and light.
- Their bodies are streamlined and extremely light.
- They have powerful flight muscles.
- Their breastbone is extended (as keel) for the attachment of large flight muscles (which move the wings up and down).
In birds, the muscles for flying act on the wings. One pair of muscles pulls the wings down and the other pair of muscles pulls the wings up. When a bird moves its wing down, it is called a ‘downstroke’. And when the bird moves its wings up, then it is called an upstroke. The birds move their wings up and down quickly by the rapid contractions and relaxation of their flight muscles. We will now describe how a bird actually flies.
Birds fly by a constant flapping (up and down movement) of their wings. During the downstroke, the raised wings of a bird go down and back [see Figures (a) and (b)]. When the wings go down, they push the air down. The air pushes up the wings with an equal force which makes the bird go up.
The backward movement of wings pushes the air in backward direction. This air pushes the wings in the forward direction and makes the bird move forward. Thus, during the downstroke of wings, the air is pushed downwards and backwards. The equal and opposite reaction force of air pushes the bird up and forward, and makes it fly.
After making the downstroke, the wings must go back up to their starting position. During the upstroke, the bird brings its wings from the lowermost position to uppermost position so that it may repeat the downstroke to keep flying [see Figures (b) and (c)]. While bringing the wings up during upstroke, the bird partly folds its wings so that the effect of air is reduced.
Movement Without Legs
We usually associate movement with legs. But all the animals do not have legs. The animals which do not have legs use only the muscles of their body for moving from place to place. Some of the animals which do not have legs are earthworm, snail, fish and snake. We will now describe how these animals move from one place to another without legs.
An earthworm does not have bones or legs. It has muscles which help to lengthen and shorten the body. An earthworm’s body consists of liquid-filled segments (or compartments) joined together [see Figure (a)]. Normally, the segments of earthworm’s body are short and fat. The earthworm’s body has two types of muscles : circular muscles and longitudinal muscles.
- When circular muscles contract, they make the segments of earthworm’s body long and thin.
- And when longitudinal muscles contract, they make the long and thin segments of earthworm’s body short and fat again.
Each segment of earthworm’s body has tiny bristles (hair like structures) on its underside. These bristles can be moved into the ground to grip the ground, or withdrawn to release the ground. Keeping all these points in mind, we will now describe how an earthworm moves from one place to another.
An earthworm moves by ‘lengthening’ and ‘shortening’ its body alternately by using the circular muscles and longitudinal muscles, respectively. The tiny bristles on the underside of earthworm’s body help in gripping the ground when a part of its body moves. We will now describe the movement of an earthworm with the help of diagrams.
(i) Figure (a) shows the normal shape of the body of earthworm when it is stationary (not moving). In this position, all the segments of earthworm’s body are short and fat. The bristles on the underside of the body are gripping the ground.
(ii) To move forward, the earthworm contracts circular muscles of the front segments due to which the front part of its body becomes long and thin, and moves ahead by some distance [see Figure (b)]. During this time, the bristles of front part of earthworm’s body are withdrawn from the ground but the back part of earthworm’s body remains fixed to the ground by the gripping action of bristles.
(iii) In the next step, the longitudinal muscles of front part of earthworm contract and make it short and fat again [see Figure (c)]. The bristles of front part now grip the ground. At the same time, the circular muscles of back part contract and make the back part of earthworm’s body long and thin [see Figure (c)].
(iv) Finally, the longitudinal muscles of back part of earthworm contract and make it short and fat, pulling the back of body forward [see Figure (d)].
In this way, earthworm completes one step of its movement and covers a small distance. By repeating these steps, an earthworm keeps on moving further. The body of earthworm secretes a slimy substance (moist, soft and slippery substance) which helps in its movement.
Snail is a very slow moving animal with a shell on its back (see Figure). The body of a snail is soft and consists of a head, a foot and a shell. The head of snail has two pairs of tentacles for catching prey. The larger pair of tentacles has little eyes in them. Shell is the outer skeleton of snail but it is not made of bones. Shell contains the internal organs of the snail. In dangerous (or unfavourable) conditions, the snail withdraws its head and foot into the shell. The snail has only one foot under its body. The foot of snail is large, flat and disc-shaped, which is made of strong muscles. It is called muscular foot.
The snail moves with the help of a large, disc-shaped muscular foot. This happens as follows : There are two sets of muscles in the foot of snail which contract and expand alternately, producing a kind of wave effect (from back to front). A series of waves in the muscles of foot make the snail move forward.
Snails are ‘gastropods’ which means ‘belly-footed animals’. The movement of snail is called ‘creeping’. In a way, the movement of a snail is similar to that of an earthworm. An earthworm, however, moves much faster than a snail.
Fish can move in water (or swim in water) because its body is specially adapted for this purpose. Fish is adapted to move in water by having a streamlined shape, flexible backbone, powerful body muscles and fins. This is explained below.
- The Fish Has Streamlined Body. The body of fish tapers at both ends (being thin in front, thick in middle, and again thin at the back) (see Figure). This body shape is called ‘streamlined’.
Due to streamlined shape, water can easily flow around the body of fish and offers least resistance to the movement of fish.
- The Fish Has Flexible Backbone. Due to flexible backbone, the fish can bend its body easily from side to side to move through water.
- The Fish has Powerful Body Muscles. The powerful body muscles of the fish on both sides help in moving its tail on both sides.
- The Fish Has Fins. The thin and flat projections on the body of fish are called fins. The fins help the fish in steering, balancing and stopping in water. The tail fin also helps in moving the fish forward in water.
We will now describe how the fish moves in water (or swims in water). In order to understand this please keep in mind that when an object pushes water in the backward direction with a certain force, then water pushes that object in the forward direction with an equal force.
A fish swims in water by moving its tail from side to side. The side to side movements of tail are brought about by the alternate contractions and relaxation of muscles on the two sides of the body of fish. The muscles contract first on one side of the body and then on the other side, making the tail move from side to side. When the fish’s tail moves from side to side, it pushes the water sideways as well as backwards. The backward push of tail on water, makes the fish move forward.
In Figure (a), the tail of fish is pushing water to right side and backwards. The effect of right side
push of tail on water is compensated by the fish by turning its head to the left side. The backward push of tail on water makes the fish move forward.
In Figure (b), the tail of fish is pushing water to left side and backwards. The effect of left side push of tail on water is compensated by the fish by turning its head to the right side. Here again, the backward push of tail on water makes the fish move forward. The net result of the beating of water with tail on both the sides is that the fish keeps on moving forward (or swimming) continuously.
The body of a snake is long and cylindrical. The snake has a long and flexible backbone which makes its body bend easily to form loops (or curves). It has also strong muscles connected to its skeleton. Snakes do not have legs, even then they move quite fast.
Snakes use their whole, long and flexible body to move from one place to another. Snakes have several ways of moving about. We will describe the most common way of movement in snakes.
A snake ‘moves forward’ by moving its body ‘sideways’ in the form of many loops, and pushing against the ground (with the underside of moving loops). In most simple words we can say that a snake moves forward by moving sideways ! We can explain the movement of a snake as follows.
A snake contracts and relaxes the muscles on the two sides of its body alternately to form many loops (or curves) in which different parts of the snake’s body are moving to the left side and right side at the same time (see Figure). Each sideways moving loop of snake’s body pushes back against the ground and gives the snake a forward push. The resultant push of all the loops of snake’s body make the snake move forward very fast.
The special name of the wave-like motion of a snake is ‘slither’. So, we can also say that the snakes slither on ground by looping sideways. Please note that a snake does not move in a straight line. This is because the sideways pushes of the loops push the snake forward as well as sideways.