1. Fish are the oldest vertebrates and live in various aquatic environments.

2. They have fins and tails, and some are big, some are small.

3. Fish eat small worms and are a common food source.

4. There are many kinds of fish, some are common, others are precious.

5. Fish have many functions and are important to people's lives.

这些句子简洁明了地介绍了鱼的基本特征和生活方式。