The incredible journey from fish to frog

Posted in Animals, Nature, Wildlife on Friday, 28 October 2011

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This edited article about amphibians originally appeared in Look and Learn issue number 850 published on 29 April 1978.

Frog, picture, image, illustration

A frog catching a butterfly, with an inset of its lifecycle

In what animal can we see, unfolded before our eyes, the story of evolution? It seems incredible to think that any one creature could lay before us such a fascinating story. Yet there is an animal whose creation parallels the development of life on our planet.

Evolutionists tell us that single-celled creatures called amoebae living in the sea were the earliest forms of animal.

Among the myriad forms that arose from this were the fishes and then the amphibians that were equally happy in or out of the water. From these there came reptiles and, eventually, mammals.

From amoeba to amphibian is a long step, yet it is one which is taken, loosely speaking, by our present-day frogs. In them, you can see millions of years of evolution taking place in a few weeks, beginning with the jelly-like egg, which we can liken to the amoeba, and passing through the stages of water-animal to amphibian.

In Britain, the female frog lays her spawn on the bottom of a pond or ditch early in March or the middle of April. The spawn consists of a mass of yellowish jelly in which are embedded black spots.

The black spots which are the eggs start to get bigger and bigger. If you look closely at the spawn you can see them turning into tiny tadpoles.

About ten days after the spawn comes to the surface, the tadpoles begin to wriggle and eventually burst their way out of the transparent jelly.

On the under surface of the newly-hatched tadpole’s head is a tiny groove where the mouth will later develop. Just below the groove is a small fold of skin called the adherer.

Immediately the tadpole leaves the spawn jelly it paddles with its tail to a piece of waterweed to which it attaches itself with its adherer.

All the time it is growing bigger and stronger. The fact that it cannot eat because it has no mouth does not worry the tadpole just now. Its body absorbed enough nourishment from its jelly to keep it alive for several days.

During these few days when it is anchored to the weed, the tadpole is quickly changing shape. Its tail grows longer, its body gets thicker, and the head begins to develop eyes and a mouth.

At the same time, gills begin to appear on the side of the head. Gills are the special organs with which fish breathe by extracting oxygen from the water.

Then the adherer disappears and the tadpole, now complete with a mouth, casts off from its piece of weed and paddles away in search of food.

Another few days, and the gills disappear inside the head and become what are called spiracles. The tadpole is now breathing internally the oxygen from the water.

The groove on the under part of the head next turns into a fully-formed mouth with funnel-shaped lips covering tiny horned teeth. With these the tadpole is able to chew up the water-fleas and other minute pond life on which it feeds.

Three weeks or so after it has hatched, the tadpole begins to grow hind legs, which start as tiny buds at the root of the tail. Ten days later the developing arms or forelimbs begin to appear.

At the same time, the various organs inside the body are changing. Lungs and other organs which will be needed when the tadpole develops into an air-breathing creature begin to develop.

These internal changes make it difficult for the tadpole to eat, so it lives on its tail, which is a kind of built-in larder. That is why a tadpole’s tail gradually disappears as its owner begins to grow more and more like a frog.

While all this is going on, the tadpole’s gills are disappearing altogether and it has to learn to breathe, poking its head up through the surface to take gulps of air.

At last the tiny frog ventures out of the water on the bank, where it hops about the long damp grass, feeding on green-fly and other small insects. These it captures with its long sticky tongue, which it flicks in and out of its mouth at an incredible speed.

The term “amphibia”, given to the class of animals to which frogs belong, comes from the Greek amphibios (“double life”), referring to the fact that such creatures live both on land and in water.

Toads, newts and salamanders, and some worm-like species found in tropical countries, also belong to this class.

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