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Chondrichthyes

Chondrichthyes 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).

animals from this group have internal fertilization and a reproduction strategy reminiscent of that seen in amniotes. They also have a relative brain development of its major divisions, reminiscent of those found in birds and mammals. Their brain weight relative to body size 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, although Holocephali and some pelagic sharks have lost it.

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 males 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. So what came first, the oral teeth or the dermal teeth, is not known for sure. 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.


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