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Part 2: Fish

Vocabulary

carbon dioxide
cold-blooded
fin
gills
operculum
oxygen

Fish Came First

500 million years or so, the first vertebrate animals were fish. The first fish did not have jaws at all, but they had backbones. A couple of the ancestors of these jawless fish are still around. One of them is the lamprey.

Soon, though, only 50 million years later, another type of fish came along. the bony fish, with hard bones. 60 million years later the bony fish divided into two types, the ray-finned fish and the lobe-finned fish.

Sharks, rays, and mantas developed at the same time from the jawless fish. This group has rubbery cartilage instead of bone.They do not have swim bladders, so they must keep swimming in order to keep from falling to the bottom.

In the diagram below, you can see how the different groups of fish evolved..

First
came
jawless fish
Then bony fish
Then sharks evolved
from jawless fish and
the ray-finned fish and lobe-

finned fish evolved from bony fish

The ray-finned group of bony fish includes the types of fish we are most familiar with, but also the eels, which are fish even though they do not look like it. There are more species of fish than of any other kind of vertebrate animal.

Fish Bodies

Ray-finned fish are built for swimming. Instead of legs, they have fan-like parts called fins. Fish also have bony skulls to protect their brains. They have backbones which protect the nerve cord running through them, and they have strong muscles attached to their bones so they can swim well. Bony fish have a swim bladder filled with air, which keeps them from sinking to the bottom. Many fish have scales, hard plates which are part of the skin. Finally, fish have gills. Gills are body parts which allow fish to breathe under water. The operculum, a bony flap, covers and protects the gills. All fish are cold-blooded, so the inside temperature of their bodies is the same as the temperature of the water around them.

Breathing With Gills

All animals must breathe in oxygen and breathe out carbon dioxide. Oxygen is a gas important for animal life, which makes up a big part of the air around us. But fish live under water. How can they get the oxygen they need?

There is oxygen in tiny bubbles throughout the water in the ocean. Fish take in this oxygen-filled water through their mouths. The water passes over the gills, which are very thin sheets. In the gills is blood, full of tiny carbon dioxide bubbles. Because the gills are so thin, the oxygen can go into the blood and the carbon dioxide goes out into the water. Then the water passes out of the body of the fish.

Life Cycle

In order for animals to survive, they must reproduce, or have young. Fish reproduce with eggs. The female fish lays eggs in the water. The male swims over them and fertilizes them. Then the eggs are left to hatch on their own. Many eggs do not hatch or are eaten by predators. Yet some survive so more fish can be born.

Homework

Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:

When did the first vertebrate animals evolve, and what were they?
Name three parts of a ray-finned fish.
How do fish breathe even though they live under water?
Do you think that a fish could tell who its parents were? How would you be able to find out?

Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.

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Go on to Part 3: Amphibians

This page last modified August 15, 2002

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