We do not know the exact number, and even our estimate are very rough. Animals range in size from no more than a few cells to organisms weighing many tons, such as blue whales and giant squid.
Most animals live in the oceans, with fewer in fresh water and even fewer on land.
You have been looking at the diversity, complexity, and characteristics of an organism in the invertebrate phyla. Now you will use some sites on the Internet to add to your knowledge of invertebrates. You must learn to say these seven levels of classification in their proper order. Explain what cellular-level organization means. Tell how medusae and polyps are different from one another.