Animals in the Chondrichthye class are known as cartilaginous fish because their flexible skeletons are largely made of fibrous cartilage tissue, instead of bone. As vertebrates, many chondrichthyans have bony spinal columns. These finned creatures breathe through gills and usually have nostrils located near the bottom surface of their heads.
The Chondrichthye class includes sharks, skates, stingrays and chimaeras. Chondrichthyans have snouts that protrude past their mouths and roughly four to seven pairs of gills. They lack the large amounts of calcium deposits found in other vertebrates, so chondrichthyans don't have hard skeletal structures in their fins or heads. However, their bodies are protected by tough skin layered with tiny rows of rigid, platelike scales.
All chondrichthyans are aquatic animals, and the majority of species swim by coordinating the movements of their bodies, caudal fins, pectoral fins and dorsal fins. The caudal fin, or tail, helps the animal propel forward, while the pectoral fins on the left and ride sides of the body are used to steer and adjust depth while swimming. One or two dorsal fins are located on a chondrichthyan's back, providing balance in the water. Some species have an additional pelvic fin and anal fin on the undersides of their bodies. Males typically have a clasper, an organ on the pelvic fin that allows the creature to fertilize a female's eggs.