All organisms of a phylum are further categorized into classes. These classes are based on very important, and more detailed, similarities. There are about 87 classes of organisms in the Animal Kingdom. Many times the classification of organisms into classes, or any other level of classification for that matter, is based on observation. There is no scientific method in organizing living things in this manner. For this reason, the number of classes, or any level of classification, could change at any moment.
The Animal Kingdom contains many, known classes of organisms. The phylum Chordata contains the following classes: mammals, birds, fish, reptiles, and amphibians. These are the most known classes of the entire animal kingdom. The major differences among these classes have to do with how organisms bear their young. Some animals give birth while others lay eggs. Some lay eggs on land while others lay eggs in water. Some that lay eggs in water might continue to live in the water after they are born and others move to live on land after they are born. Humans are in the Mammal Class because we give birth to our young and we drink milk as a baby. Other mammals such as horses and whales share this characteristic with humans.
The Arthropods Phylum contains two important classes: arachnids and insects. Arachnids include spiders. Many people believe that spiders are insects but they are actually in a different class. Spiders differ from insects in a few ways, namely they have 8 legs while insects have 6, they do not have wings or antennae like insects, and they have two body segments while insects have three.