Thelodonti ├─Placodermi └─Eugnathostomata ├─Chondrichthyes │ ├─Eugeneodontida │ └─Chondrichthyes (Crown) │ ├─┬─Cladoselachida │ │ └─Symmoriida │ │ ├─Symmoriidae │ │ └─Holocephali │ └─Elasmobranchii │ ├─Xenacanthida │ └─Ctenacanthiformes │ ├─Ctenacanthidae │ └─Euselachii │ ├─Hybodontiformes │ └─┬─Synechodontiformes │ └─Neoselachii └─Teleostomi
The Chondrichthyes or "cartilaginous fishes" are so called because their internal skeletons are composed of cartilage (the stiffening substance in your nose and ears), reinforced with small bone plates. They have small tooth-like scales covered with an enamel-like layer. In effect, a tooth-like skin! there is no swim-bladder, lift is provided internal oil bodies that are lighter than water and by their streamlined shape as it moves through the water.
The sense of smell and the lateral line system (pressure sensitive organs that can pick up vibration and movement in the water) are both well-developed, as befits a predator that seeks out prey in murky water.
As with mammals, the male fertilises the female internally. Instead of a penis, a pair of erectile claspers on the inner edge of the pelvic fins are used.
There are over 950 recent species, and many more fossil forms. The group is divided into two very different subclasses, which seperated very early on: the Elasmobranchii (sharks, skates and rays) and the Holocephali (the chimaeras, such as the ratfish and elephant fish). The living elasmobranchs (Neoselachii) are covered in a later section.