The word "locomotion" means "moving from place to place". It comes from two Latin words: "locus", meaning "place" and "motio" meaning "movement". Animals can move from place to place in water, in air, or on land. Such movement does require work and their bodies must be lifted or moved forward or both.
Even humans can notice this easily. When you walk, you feel the weight of your body. As you lift each foot, you work against the downward pull of gravity. When you swim, the water supports your weight almost entirely. The downward pull of gravity is balanced by the upward lift of the surrounding water. However, you will have to work to push through the water as it resists your moving body. On land in, air resists your movement very little. You can even strongly notice the resistance. Therefore, animals that live in water work mainly to overcome the resistance of their surroundings. Animals that live out of water work mainly to overcome the pull of gravity. Locomotion is naturally very different in air and in water.
A fast-swimming fish, such as a mackerel, has an ideal shape for moving quickly through water. The body tapers at the front and tail. The shape meets very little resistance from surrounding water since water streams smoothly over the surface of the body. It is also the shape of large sea mammals such as whales and dolphins. This is the shape men have chosen to base the submarine design on. The streamlined shape is necessary for rapid movement in water because it gives the much needed stiffening and power.
The force of the forward push depends on the size, shape and slant of the tailfin whilst the animal's speed through the water is determined by its body size. A dolphin 6 feet long can easily swim at 20 mph. Its body shape is streamlined like that of a fast-swimming fish, but its body and tail speak up and down rather than side to side. The surface of a dolphin's body does meet some resistance from the water and this is why huge whales about 100 feet long swim no faster than the smaller dolphins. Exception when the animal is attacking its spray or escaping an enemy, to swim at high speeds demands more effort than the body can afford.
Others sea animals move through water much more slowly. Worms and snails glide along the bottom while turtles and other animals paddle. Jellyfishes move by some sort of jet propulsion. The squid, octopus and the clam swim by somewhat the same method as well.
On land most kinds of animals move from place to place a means of legs. Legs work against the pull of gravity by lifting and supporting the weight of the body to carry it forward. For rapid locomotion over the ground, a few pairs of longer legs seem better than lots of short legs. A spider can move across the table faster than a millipede can and a mouse can move faster than a spider. The fewer and longer the legs are, however, the better the body must be balanced. In an insect, the body must be carefully balanced so all its legs support the middle part of the body. An insect moves only three legs at a time because the other three legs are used for support. And an insect has its weight equal in both the front and behind of the body. Although this is good for balance, this arrangement is not as good for fast local motion as in vertebrates.
Four legs support most land vertebrates and move it from place to place. In some animals like the salamander, the legs are at the sides so the animal wriggles like a fish. When standing, some vertebrates place their body weight on their front legs and some place it on their hind legs. Others also place it evenly on both pairs of legs. In order not to fall down when walking, a four-legged animal must always have the other 3 feet on the ground. No foot is lifted off the ground without the center of the body weight is spread over the other 3 feet. For instance, if movement movements with the right front leg, the left rear leg steps next. Then the left front leg moves, followed by the right rear leg. This pattern is the same for a turtle, dog, or a human infant on all fours. By walking this way, a four-legged animal can stop suddenly at any moment without falling down