Chondrichthyes

The Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. They are divided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaera, sometimes called ghost sharks).

Characteristics
Animals from this group have a brain weight relative to body size that comes close to that of mammals, and is about ten times that of bony fishes. There are exceptions: the mormyrid bony fish have a relative brain size comparable to humans, while the primitive megamouth shark has a brain of only 0.002 percent of its body weight. One of the explanations for their relatively large brains is that the density of nerve cells is much lower than in the brains of bony fishes, making the brain less energy demanding and allowing it to be bigger.

Their digestive systems have spiral valves, and with the exception of Holocephali, they also have a cloaca.

As they do not have bone marrow, red blood cells are produced in the spleen and special tissue around the gonads. They are also produced in an organ called Leydig's Organ which is only found in cartilaginous fishes, although some have lost it. Another unique organ is the epigonal organ which probably has a role in the immune system. The subclass Holocephali, which is a very specialized group, lacks both of these organs. Originally the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and pubioischiadic bars evolved. In rays, the pectoral fins have connected to the head and are very flexible.

A spiracle is found behind each eye on most species.

Their tough skin is covered with dermal teeth (again with Holocephali as an exception as the teeth are lost in adults, only kept on the clasping organ seen on the front of the male's head), also called placoid scales or dermal denticles, making it feel like sandpaper. It is assumed that their oral teeth evolved from dermal denticles which migrated into the mouth. But it could be the other way around as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth (as do probably Atherion elymus, another bony fish). This is most probably a secondary evolved characteristic which means there is not necessarily a connection between the teeth and the original dermal scales. The old placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown which of the dermal or oral teeth evolved first. Neither is it sure how many times it has happened if it turns out to be the case. It has even been suggested that the original bony plates of all the vertebrates are gone and that the present scales are just modified teeth, even if both teeth and the body armour have a common origin a long time ago. But for the moment there is no evidence of this.

Taxonomy

 * Class Chondrichthyes
 * Subclass Elasmobranchii (sharks, rays and skates)
 * Superorder Batoidea (rays and skates), containing the orders:
 * Rajiformes (common rays and skates)
 * Pristiformes (Sawfishes)
 * Torpediniformes (electric rays)
 * Superorder Selachimorpha (sharks), containing the orders:
 * Hexanchiformes Two families are found within this order. Species of this order are distinguished from other sharks by having additional gill slits (either six or seven).  Examples from this group include the cow sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
 * Squaliformes Three families and more than 80 species are found within this order. These sharks have two dorsal fins, often with spines, and no anal fin.  They have teeth designed for cutting in both the upper and lower jaws.  Examples from this group include the bramble sharks, dogfish and roughsharks.
 * Pristiophoriformes One family is found within this order. These are the sawsharks, with an elongate, toothed snout that they use for slashing the fishes that they then eat.
 * Squatiniformes One family is found within this order. These are flattened sharks that can be distinguished from the similar appearing skates and rays by the fact that they have the gill slits along the side of the head like all other sharks.  They have a caudal fin (tail) with the lower lobe being much longer in length than the upper, and are commonly referred to as angel sharks.
 * Heterodontiformes One family is found within this order. They are commonly referred to as the bullhead, or horn sharks.  They have a variety of teeth allowing them to grasp and then crush shellfishes.
 * Orectolobiformes Seven families are found within this order. They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the largest of all fishes, the whale sharks.  They are distinguished by having barbels at the edge of the nostrils.  Most, but not all are nocturnal.
 * Carcharhiniformes Eight families are found within this order. It is the largest order, containing almost 200 species.  They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks.  They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
 * Lamniformes Seven families are found within this order. They are commonly referred to as the mackerel sharks.  They include the goblin shark, basking shark, megamouth, the thresher, mako shark and great white shark.  They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes contains the extinct Megalodon (Carcharodon megalodon), which like most extinct sharks is only known by the teeth (the only bone found in these cartilaginous fishes, and therefore are often the only fossils produced) and a few vertebrae.  The largest  of the teeth of this shark can measure (up to more than 7 inch in length) and through modern research, it has been determined that this shark could exceed 50 feet in length.


 * Subclass Holocephali (chimaera)