英文科學(xué)讀本 第三冊(cè)·Lesson 19 A Snake

Lesson 19 A Snake

I wish Norah could have been in our class today, Will, don't you? said Fred. "I want to tell her all about our lesson, but I don't think we could make her understand these animals without pictures of them. I wish we had some of the pictures that teacher showed us at school."

Just then his father came in with something under his arm. "Look, children," said he, "what I have got for you. I have been very pleased to see you take so much interest in your lessons, and I have bought you this beautiful book of Natural History. It is full of pictures of animals, and I am sure it will be very useful to you."

How they shouted, and clapped their hands, and kissed their good kind father! It was really a sight worth seeing.

You are a dear good father, said Fred, while the tears sparkled in his eyes. "This is the very thing we wanted. Now we can get along beautifully."

Go on then, said his father, "and as I have a little time to spare, I will sit and listen to you."

Fred began by turning to some of the pictures of snakes. He pointed out the strange form of this new kind of animal—a creature with a very long body, but without the least sign of legs or limbs of any sort.

But you must not think, he added, "that these are sluggish, helpless animals, because they have no limbs. They are all very quick and nimble in their movements. They are equally at home on the ground or among the branches of trees, for they climb trees and dart from branch to branch with the greatest rapidity."

But I can't see how they can move at all, said Norah, "without limbs. Other animals use their limbs for moving about."

Teacher made us understand it all today, replied Fred, "and I think I can make it clear to you."

Look at this picture. It is the skeleton of the snake. That great bone which runs through the whole length of the animal from head to tail is the backbone. Teacher showed us that this is not a good name, for it is really a string of separate bones, and they are joined loosely together in a very wonderful way.

Each bone has a little round knob or ball in front, and a hollow cup or socket behind. The cup of one ball fits into the socket of the next and forms what is known as a 'ball-and-socket' joint. Such a joint makes the body very flexible, and enables the creature to bend and twist easily in all directions.

Now let us look at the short curved bones arranged in pairs on either side of the backbone. These are the ribs. I daresay you know that a man has a backbone and twelve pairs of ribs. Look what a number of ribs this snake has. Some snakes have as many as 300 pairs of ribs.

Teacher next pointed out to us that these ribs are all jointed to the backbone by ball-and-socket joints, and that the lower end of each rib is free to move. They can move easily backwards, forwards, sideways—in every direction.

We will next turn for a moment to the picture of the snake itself. The body, you see, is covered with scales, the one in front overlapping the one behind, so as to leave the hinder edge of all of them free.

The scales are hard and horny, and those on the under part of the body are larger than the rest. These are joined to the lower ends of the ribs.

Now, Norah, for your surprise. The snake moves about with these scales as if they were legs. The ribs can move in any direction. When the snake wishes to move, therefore, it advances its ribs. These carry with them the under scales. The scales take a firm hold of the ground or tree on which they rest, and the body is drawn forward with a gliding movement. Isn't that all very wonderful, sister?

SUMMARY

Snakes have no limbs. They move on their ribs. They have many pairs of ribs. The skeleton is jointed with ball and socket joints. The body is covered with scales.