英文科學(xué)讀本 第四冊(cè)·Lesson 13 Vertebrates and Invertebrates

Lesson 13 Vertebrates and Invertebrates

I want you to think about some of the animals of our former lessons, said Mr. Wilson.

Cats, dogs, sheep, horses, birds, snakes, frogs, and fishes are all alike in one respect, and all differ from such animals as bees, butterflies, beetles, and spiders in that same one respect. You know you have a backbone—you can feel the bony knobs or projections down the middle of your back—and you have lately been learning many things about the hard bony skeleton, which supports and gives shape to your body. We ourselves are like the cat, dog, sheep, bird, snake, and fish in this respect, for they too have a backbone and an internal bony skeleton. Butterflies, beetles, spiders, and hosts of other animals have no backbone at all. This distinction enables us to make our great classification of the animal kingdom. We arrange all animals in two groups—those which have a backbone, those which have not a backbone.

If you think for a moment about this backbone, you will be able to tell me why the name is not a good one.

It is not a single bone all in one piece, sir, said Willie, "it is a string or column of separate bony rings, which fit closely together."

I think, too, you can tell me how many of these bony rings you have in your own backbone, said Mr. Wilson.

I have thirty-three now, sir, but when I become a man there will be only twenty-six, because some of the bones grow together.

Quite right. Most of the animals we meet with have more of these separate pieces in their backbone than we have. You, of course, remember that snakes have a very great number. What is the object of so many separate pieces?

It is to give freedom of movement, sir, said Fred, "for although each bone can move only to a slight extent, this long chain of bones gives the whole column great flexibility."

Very good, Fred. Now tell me the name which we give to these separate bones.

We call them vertebrae, sir.

Right. You must try and remember that the name comes from the Latin word verto, which signifies 'I turn,' and then you will see the reason why it is given. By means of these vertebrae we and other animals can turn, move, or bend our backs and necks freely. As the backbone is made up of vertebrae, it is sometimes called the vertebral column, and all animals which have a backbone are called vertebrate animals or vertebrates. They form the first sub-kingdom in the animal world. The other sub-kingdom comprises all animals which have not a backbone. They are called invertebrate animals or invertebrates. 'In' means 'not.'

You called these vertebrae bony rings, Fred. Why did you give them this name?

Each vertebra, sir, is pierced through from top to bottom with a rounded hole, and as the bones fit together, these holes also fit to each other, so as to form one continuous tube through the column. This tube is called the vertebral canal, and sometimes the spinal canal. The spine is another name for the backbone.

All this is very good, Fred, said Mr. Wilson. "Can you tell me anything more about that canal?"

The upper part of the canal, sir, opens out into the cavity of the skull. The brain fills the skull, and part of it, called the spinal cord, passes out from the skull and runs through the spinal canal. It is the great nerve of the body.

Just so, and now remember, said Mr. Wilson, "that this arrangement is not only true as regards ourselves, but for all vertebrate animals. Every animal that has a backbone has also a skull, containing a brain and a nerve-cord running from it through the spinal cavity.

Now I want to notice how the vertebrae of different animals are joined. Can you tell me how our own are joined?

Each vertebra rests upon a pad of elastic cartilage, sir, said Fred. "These pads allow a certain amount of movement, but prevent one bone from grating against another."

Quite right, Fred, and this is the arrangement in most of the vertebrate animals. Now let us think for a moment about the vertebrae of the snake. How are they joined?

They are joined by ball-and-socket joints, sir; these joints give the creatures their extreme flexibility, and their rapid gliding movement. In fishes each vertebra has a sort of cup or socket on either side, and it is only the rims of these cups which fit together, the hollow being filled with a sort of fluid."