Chondrichthyes

Chondrichthyes (from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).

Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi.

Skeleton
The skeleton is cartilaginous. The notochord, which is present in the young, is gradually replaced by cartilage. Chondrichthyes also lack ribs, so if they leave water, the larger species' own body weight would crush their internal organs long before they would suffocate.

As they do not have bone marrow, red blood cells are produced in the spleen and the epigonal organ (special tissue around the gonads, which is also thought to play a role in the immune system). They are also produced in the Leydig's organ which is only found in cartilaginous fishes, although some do not possess it. The subclass Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organ.

Appendages
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. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. Another exception are the electric rays, which have a thick and flabby body, with soft, loose skin devoid of dermal denticles and thorns.

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.

One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.

Body covering
Chondrichthyes have toothlike scales called denticles or placoid scales. Denticles provide two functions, protection, and in most cases streamlining. Mucous glands exist in some species as well.

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 does, probably, Atherion elymus, another bony fish). This is most likely 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 armor have a common origin a long time ago. However, there is no evidence of this at the moment.

Respiratory system
All Chondrichthyes breathe through 5-7 gills, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills whilst demersal species can actively pump water in through their spiracles and out through their gills. However, this is only a general rule and many species differ.

A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the Nurse Shark (Ginglymostoma cirratum), to extended and slit-like, such as found on the Wobbegongs (Orectolobidae). Many larger, pelagic species such as the Mackerel Sharks (Lamnidae) and the Thresher Sharks (Alopiidae) no longer possess them.

Biology
Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth; however, some Chondrichthyes do guard their eggs.

Phylogeny
Subphylum Vertebrata └─Infraphylum Gnathostomata ├─Class Placodermi — extinct (armored gnathostomes) └Microphylum Eugnathostomata (true jawed vertebrates) ├─Class Chondrichthyes (cartilaginous fish) └─(unranked) Teleostomi (Acanthodii & Osteichthyes) ├─Class Acanthodii — extinct ("spiny sharks") └Superclass Osteichthyes (bony fish) ├─Class Actinopterygii (ray-finned fish) └─Class Sarcopterygii (lobe-finned fish) Note: lines show evolutionary relationships.

Taxonomy
The extant members of the Chondrichthyes are the sharks and rays, belonging to the subclass Elasmobranchii, and the chimaeras, belonging to the subclass Holocephali. Nelson's 2006 Fishes of the World (ISBN 0471250317) arranges the class as follows:


 * Subclass Elasmobranchii
 * †Plesioselachus
 * †Order Squatinactiformes
 * †Order Protacrodontiformes
 * †Infraclass Cladoselachimorpha
 * †Order Cladoselachiformes
 * †Infraclass Xenacanthimorpha
 * †Order Xenacanthiformes
 * Infraclass Euselachii (sharks and rays)
 * †Order Ctenacanthiformes
 * †Division Hybodonta
 * †Order Hybodontiformes
 * Division Neoselachii
 * Subdivision Selachii (modern sharks)
 * Superorder Galeomorphi
 * Order Heterodontiformes (bullhead sharks)
 * Order Orectolobiformes (carpet sharks)
 * Order Lamniformes (mackerel sharks)
 * Order Carcharhiniformes (ground sharks)
 * Superorder Squalomorphi
 * Order Hexanchiformes (frilled and cow sharks)
 * Order Echinorhiniformes (bramble sharks)
 * Order Squaliformes (dogfish sharks)
 * †Order Protospinaciformes
 * Order Squatiniformes (angel sharks)
 * Order Pristiophoriformes (sawsharks)
 * Subdivision Batoidea (rays)
 * Order Torpediniformes (electric rays)
 * Order Pristiformes (sawfishes)
 * Order Rajiformes (skates and guitarfishes)
 * Order Myliobatiformes (stingrays and relatives)
 * Subclass Holocephali
 * †Superorder Paraselachimorpha
 * †Order Orodontiformes
 * †Order Petalodontiformes
 * †Order Helodontiformes
 * †Order Iniopterygiformes
 * †Order Debeeriiformes
 * †Order Eugeneodontiformes*
 * Superorder Holocephalimorpha
 * †Order Psammodontiformes*
 * †Order Copodontiformes
 * †Order Squalorajiformes
 * †Order Chondrenchelyiformes
 * †Order Menaspiformes
 * †Order Coliodontiformes
 * Order Chimaeriformes (chimaeras)

* position uncertain