As you have been learning, scientists classify animals into groups that have things in common with one another. The very first thing animals are separated by is whether or not they have a backbone. Animals with a backbone are known as vertebrates. Animals without a backbone are called invertebrates. From these groups come even smaller groups of similar animals.
For instance, animals with a backbone are further divided into reptiles, amphibians, birds, fish, and mammals. There are distinguishing characteristics between all of these groups. Mammals such as humans, bears, and dogs have hair and give birth to live young, but birds have feathers and lay eggs. Fish live in the water all their lives, while amphibians like frogs start in the water and then grow up to move onto the land. Reptiles such as crocodiles and turtles are air-breathing and covered with scales.
Invertebrates include things like worms, mollusks, arthropods, and sponges. Each of these groups also has distinguishing characteristics. Believe it or not, there are many different types of worms. Some don’t have any organs, while some have full digestive and nervous systems. Mollusks can be soft-bodied like the octopus or have a shell like a snail. Sponges have pores throughout their bodies. Arthropods are the largest group of animals. This group includes all insects, and even crabs and lobsters – all animals with jointed legs and an exoskeleton.