In general the Invertebrates are animals without a backbone; that is, they do not have an internal supporting skeleton of bone, as does the dog or cat. Compared with mammals or birds, they are all small and some are so very tiny that they can be seen only with a very powerful microscope. Most of them live in the water or in the mud or sand under the water. Hence the best place to get acquainted with them is along the seashore or near some lake or stream.
There are several different groups of Invertebrates and between these groups there are greater differences of structure than there is between a horse and a hummingbird. The principal groups are:
1. The Protozoa, or one-celled animals (nearly all microscopic).
2. The Sponges.
3. The Jellyfishes, Sea-anemones, and Corals.
4. Worms of several groups.
5. Starfishes, Sea-urchins, and Sea-cucumbers.
6. Segmented Worms.
7. Crabs, Lobsters, etc.
8. Oysters, Snails, and Octopi.