Sorting Out Taxonomy: Class

​“Class” is placed between the “phylum” and “order” classification categories. Organisms in a class have more characteristics in common than they do with the rest of the species in the same phylum.

Not all scientists agree with the following classification scheme, and it is subject to change upon receipt of new information. 

For demonstration purposes, we will continue in Kingdom Animalia and Phylum Chordata to show how this Phylum is split into classes.

The Kingdom Animalia, the animals, contains 35 phyla in the world. In Illinois, 17 of these phyla are represented.

Recent taxonomic work shows that the Phylum Chordata, the vertebrates, contains 19 classes, six of which are represented in Illinois.

The organisms grouped in a class have more similar characteristics than they do with species in other classes in the same phylum.

The Class Cephalaspidomorphi contains the lampreys and jawless fishes. They do not have jaws or paired fins. The body is cylindrical, and its skeleton is made of cartilage.

WAFChestnutLamprey-UT.jpgchestnut lamprey (Ichthyomyzon castaneus)
Photo © Uland Thomas

The bony fishes compose the Class Actinopterygii. They have a skeleton made of bone. The gill openings are covered by a flap. The pectoral and pelvic fins are paired. They are found in both fresh and salt water.

WAFRedearSunfish-ISEE.jpgredear sunfish (Lepomis microlophus)
Photo © Isaac Szabo/Engbretson Underwater Photography

The amphibians, such as the salamanders and frogs, are classified in the Class Amphibia. Most of its members have four feet. Most amphibians must return to water to reproduce. The aquatic larval stage is the tadpole. Most amphibians must live in a moist environment. The skin and lungs are used for gas exchange in the adults while the larvae breathe with gills. Most amphibians produce toxins in the skin. Amphibians do not have scales, feathers or hair.

WASCaveSalamander-SB.jpg
cave salamander (Eurycea lucifuga)
Photo © Dr. Todd Pierson

The Class Reptilia includes the reptiles, lizards, snakes and turtles. These organisms develop from an amniotic egg, which is either deposited on land or retained within the female for the extent of its development. The egg provides everything the embryo needs to develop. Those reptile eggs deposited on land have a leathery shell. Fertilization is internal. Reptiles are covered in scales, and their skin is dry.

WASNEasternMilksnake-TP.jpg eastern milksnake (Lampropeltis triangulum triangulum)
Photo © Dr. Todd Pierson

Birds are the members of the Class Aves. Birds are the only living organisms with feathers. They are endotherms, maintaining a nearly constant body temperature. They have a hard bill but no teeth. A gizzard that functions to grind food is present in the digestive tract. Fertilization is internal. The eggs have a hard shell and are deposited and incubated in a nest.

WABWoodDuckMale.JPG wood duck (Aix sponsa) male
Photo © Illinois Department of Natural Resources

The Class Mammalia contains the mammals. Mammals are endotherms, maintaining a nearly constant body temperature. Most mammals have young born after developing inside the mother's body in a special organ called a uterus. After birth, the young are fed with milk produced in the female’s mammary glands. A mammal has a large and complex brain.

WAWMEasternFoxSquirrel.JPGeastern fox squirrel (Sciurus niger)
Photo © Illinois Department of Natural Resources