Classes of fish

There are three classes of fish. Fish that don't have jaws belong to the jawless fish class. Fish that have skeletons made of tough, flexible cartilage belong to the cartilage fish class. Most of the fish you're familiar with belong to the bony fish class. These fish have skeletons made of bone.

Jawless fish such as lampreys and hagfish don't look like fish. They have no jaws with which to open and close their mouths. Jawless fish have no scales and their skeletons are made of cartilage. Lampreys are parasites that live in fresh and salt water. Their round mouths are filled with small curved teeth. With these teeth lam­preys cut holes in other fish's bodies. Lampreys attach themselves and suck the body liquids and blood out of the holes. Hagfish live only in the oceans. They have fewer teeth than lampreys. But hagfish do have slit-like mouths that help them cut holes in other fish. Once holes are made, the hagfish crawl inside and feed.

Like the jawless fish, the cartilage fish also has skele­tons made of cartilage. Unlike the jawless fish, the car­tilage fish have jaws and scales. Sharks are one example of cartilage fish. They are the fastest-swimming fish.

The mouths of the sharks are filled with rows of sharp teeth that slant backward. These teeth help sharks hold their prey. As sharks' teeth are lost or broken, other teeth move forward to replace the lost ones.

Another type of cartilage fish, the rays, has a flat body and lives on the ocean bottom. Some rays have strong jaws that crush the shells of clams and oysters. Other rays, such as the stingray, have poisonous spines near their tails. Their poison causes pain and even death to their prey.

Most fish known today belong to the class of bony fish. Bony fish are the fish you know best – bass, flounder, cod, sole, trout, and so on. Bony fish are covered with scales that are smooth and slimy. The slimy covering helps fish glide through the water. It also helps protect them from parasites. The scales of bony fish grow larger as the fish grow older. Growth rings on the scales show the age of the fish.

Most bony fish have an air bladder which enables them to swim at a certain depth. Gases from the blood move into the air bladder. As the bladder fills with gases, the fish rises in the water. If gases move out of the bladder, the fish will sink deeper in the water.

Many bony fish have well-developed fins which make these fish excellent swimmers. Sailfish and tuna can swim for long distances at fast speeds. Other fish have different kinds of fins. Flying fish have winglike fins that help the fish to leap above the water. The walking cat­fish has muscular fins that help it move on the land. In­stead of separate fins, eels have fins that are continuous and joined.

Most fish reproduce by external fertilization. This means that eggs and sperm join outside the bodies of the fish. Female fish usually release their jelly-coated eggs in the water. Some species lay eggs in nests made from weeds, twigs, or mud. Male fish release sperm to fertilize the eggs. Most fish lay thousands and even millions of eggs at a time. Why aren't the waters overcrowded with fish? First, not all of the eggs are fertilized. Second, other eggs are eaten by predators before they hatch. Some of the young fish will be eaten before they become adults.

Bony fish provide food for the world's population, that is high in protein and low in fat. Some countries, such as Norway and Japan, rely on fish as their main food. Fish are also used to make animal feed and glue. Sharks' skins are used to make shoes. Oil from fish is used in making paints and varnishes and in tanning leather.

Exercise 1

Find in the text English equivalents of the following words and word combinations:

бесчелюстные рыбы; хрящевые рыбы; костные рыбы; щелевидное ротовое отверстие; удерживать добычу; быть покрытым шелухой; оплодотворение вне организма; оплодотворять яйца; с высоким содержанием белка и низким содержанием жиров.

Exercise 2

Complete the sentences:

1. Fish that don't have jaws belong to …

2. Fish that have skeletons made of tough, flexible cartilage belong to …

3. Fish that have skeletons made of bone belong to …

4. Sharks are one example of …

5. Most fish reproduce by …

Exercise 3

Answer the questions:

1. What are the major classes of fish?

2. What are the examples of jawless and cartilage fish?

3. How do bony fish look like?

4. How do most fish reproduce?

5. Where is fish used?