Cartilage

Chondrichthyes are called cartilaginous fish. You can see the clear difference from their cartilage frame. Sharks, skates and rays are the main examples of Chondrichthyes fish. This type of fish usually does not have a connection between the upper jaw and skull. It gives them great difficulty as well to move it independently. The main structure of Chondrichthyes fish is comprised of ten cartilage parts. Chondrichthyes fish also have eyelids to protect their eyes and at the same time, Chondrichthyes fish usually do not have ribs and bone marrow. The dermal dental structure is also slightly different than Osteichthyes fish. Chondrichthyes fish and their red blood cells produced in the spleen as the two lobes of the fin are not equal in size. The size of the two lobes of the fin is important as they give them sharp mobility in the water. The fin is parallel to the longitudinal axis of the main body in Chondrichthyes fish. Chondrichthyes' fins that are parallel to the longitudinal axis of the main body basically helps them to balance and move fast in the water. Chondrichthyes are often called living fossils while many scientists believe that their evolution is in progress for the past 420 million years

Read more at: Difference between Chondrichthyes and Osteichthyes http://www.stepbystep.com/difference-between-chondrichthyes-and-osteichthyes-101533/