Class- Chondrichthyes Class- Osteichthyes 1. The mouth is positioned ventrally and the body is covered with placoid scales. 1. The mouth is positioned apically and body is covered with cycloid or ctenoid or ganoid scales 2. Gill slits, usually five pairs, open sepa­rately and directly to the exterior. 2. Gill slits, usually four pairs, open in a gill chamber. 3. The endoskeleton is cartilaginous. 3. The endoskeleton is bony with amphicoelous vertebrae. 4. Operculum always absent.Examples- Sharks and chimaeras. 4. Operculum with branchiostegal mem­brane always present.Examples- Lungfish and coelacanth