Fish
There are mainly 3 classes.AGNATHA,CHONDRICHTHYES and OSTEICHTHYES.
You probably know all of the characteristics that are in bold. If you know this much, you know enough to classify any animal you see! Learn these characteristics if you don't already know them. Just think of your favorite example and the characteristics become very easy to remember. The other information gives you more characteristics that scientists use to classify animals into their respective class.
1. Fish are ectothermic, aquatic vertebrates.
2. Their skin is generally covered with scales.
3. Their limbs are modified into fins for swimming.
4. They breathe with gills.
5. They lay eggs that must be in water.
You probably knew most of this already, didn't you? The term "ectothermic" may be new to you. It means "external temperature" in Latin. The simpler term is "cold-blooded", that is, it has a body temperature equal to the surrounding environment. You know that fish are cold-blooded if you ever picked one up: they are as cold as the water they were swimming in. Even if you go swimming and are shivering with cold, you are always warmer than the water. "Aquatic" means that they spend their lives in the water.
