Class

The term class refers to one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical and range in order of size. Class is the third largest group and is located between phylum and order. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species.

Members of a class have more characteristics in common than do members of a phylum. The following example compares animals that are included in phylum with those in class. In the former are included mammals, reptiles, and birds, yet each of these are placed in a different class. Humans and mice belong to the class Mammalia because they are characterized by having hair or fur on their bodies and feeding milk to their young. Lizards and snakes belong to the class Reptilia since they are covered with scales and do not feed milk to their young. Sparrows and eagles belong to the class Aves because they have feathers on their bodies and do not feed milk to their young. All, however, belong to the same phylum Chordata since they all have a vertebrae or backbone.

In addition to these obvious differences between classes, such as body covering and the way the young are nurtured, there are more subtle ways of determining an organism's correct class. One of these is called comparative biochemistry. This process compares the deoxyribonucleic acid (DNA) of different organisms to determine if their genetic information is the same. Since the grouping by class is near the midpoint of the seven groupings, the organisms that will remain and be lumped in the next group, order, will have even more in common with each other.

[See alsoClassification; Family; Genus; Kingdom; Order; Phylum; Species ]