Why there are no three-year animals?
What is common in the appearance of a dog, cockroach and rainworm? Try mentally to draw a line from the head to the back of the body, and you can easily split the body to the right and left half. In the imagination there are two semi-windows, each of which will have one ear, one eye, two legs and a narrow tail. Two semi-cancer: one mustache, one eye, three legs. And in the case of the worm, two completely identical worms are obtained, each of which is thinner of the initial.
The same can be mentally done with other animals: goat, chicken, frog, dip, snail and even dinosaur. Everyone will have almost the same right and left half.
Of course, small differences are possible. For example, a direct horn can get one semi-chamber, and the other is a curve. Or a frog, one back leg will be slightly shorter than another, and the total identity of the halves will not work. It’s not so important. The main thing is that most of the multicellular animals have a common property – bilateral (or bilateral) symmetry.
Experts believe that the first bilateral symmetric animals appeared on our planet long ago – more than 550 million years ago. These multicellular organisms with the right and left halves did not have. They crawled on the sea bottom or algae stems. Their even more ancient ancestors led the attached lifestyle – sat at the bottom and fed on organic substances or microscopic algae, which were obtained from sea water. Such animals also had symmetry, but radial, like, for example, in jellyfish.
Then some radially symmetrical appeared wrong descendants that began to move freely. They got an advantage: when you go to search for food, it is more likely to quench the hunger than when you sit and wait. Due to crawling along the bottom of these ancient organisms, the front and rear ends of the body and bilateral symmetry were formed. These qualities turned out to be extremely helpful during movement. After all, if the right and left side of the body differ greatly from each other, they are far away, and you will spin in one place.
Subsequently, the ancient marine sliders gave rise to most groups of animals, which now inhabit the land. Therefore, their overall property – bilateral symmetry – inherited. From here it follows that the legs of most animals should be an even number.
However there is no rules without exception. And now there are animals who are not peculiar to bilateral symmetry. They are rather radially symmetrical as those ancient sitting organisms. This, for example, jellyfish, acti, sea hedgehogs and starfishes.
In addition, some, although they have an even number of legs, can use three supports for body fixation in space. For example, a kangaroo when it stands, relies on two hind legs and one thick tail. Approximately the same drawn: rests on a tree trunk with a paws and tail so as not to move down during an energetic work with a strong beak. But this support is enough for them.