Chondrichthyes

Class Chondrichthys includes all the cartilaginous fish. There are about 970 living species placed in this class. All have a cartilaginous endoskeleton, though extensively calcified; however, bone is absent from the entire class. Absence of bone is a derived character as Chondrichthyes descended from bony ancestors. However, phosphatase tissue is found in teeth, scales and spines. 

1.0Characteristics of Chondrichthyes

• Body fusiform or spindle shaped; mouth is ventral; caudal fin in sharks and rays is heterocercal; that is, the vertebral column turns upwards and extends into the dorsal lobe of the caudal fin  and diphycercal in chimeras, that is, a tail that tapers to a point and vertebral column extends to the tip without upturning ; paired pectoral and pelvic fins. 
• Skin has placoid scales or may be naked. Notochord present but reduced, vertebrae present and separate from the notochord; jaws with polyphyodont teeth (teeth that are replaced several times as they wear out), these teeth are modified placoid scales. Placoid scales have a dentine core covered by enamel. 

• Stomach large J shaped, spiral / scroll valve in intestine, liver bilobed, large and oil filled.
• Opisthonephric kidney high concentration of urea and trimethylamine in the blood; rectal gland present. 
• Brain well-developed with 10 pairs of cranial nerves; sense of smell, vibration reception, vision and electroreception well-developed; three pairs of semicircular canals present. 
• Sexes separate, fertilization internal, males have pelvic fins with claspers to assist in fertilization; oviparous or viviparous; no larval stage.
• Five to seven pairs of gills leading to exposed gill slits, no operculum; no swim bladder or lungs; heart two chambered with sinus venosus, and conus arteriosus, atrium, ventricle and, single circulation.

2.0Classification of Chondrichthyes

• Class Chondrichthyes is broadly divided into two subclasses Elasmobranchii and Holocephali. 
• Holocephali has four gill slits with a gill cover (an operculum), while Elasmobranchs have five to seven external gill slits with no gill cover.

Subclass Elasmobranchii 

• They are marine animals with stream lined body.
• Endoskeleton is made up of cartilage (Cartilaginous endoskeleton).
• Notochord is persistent through life.
• Exoskeleton is present over the skin containing placoid scales.
• Teeth are modified placoid scales, which are backwardly directed.
• Mouth is located ventrally.
• Jaws and teeth are present. Jaws are very powerful. These animals are predaceous.
• Gill slits are separate and without operculum (gill cover), 5 to 7 pairs of gills are present.
• Due to the absence of air bladder, they have to swim constantly to avoid sinking.
• A spiral valve or scroll valve is found in intestine (to increase surface area).
• Cloacal aperture is present.
• There is a special structure at the dorsal surface of head in these fishes, which is called "Ampulla of Lorenzini", this works as electroreceptor.
• Tail is of heterocercal type.
• Genital ducts open into cloaca.
• In males pelvic fins bear claspers (Copulatory organs).
• Fertilization is internal.

• E.g.- Carcharodon – Great white shark
• Pristis :- Saw - fish
• Trygon :- Sting ray - Its dorsal fin has poisonous spines.

Subclass Holocephali

• No spiracle present; gill slits are covered by a cartilaginous opercular flap creating a single external respiratory opening on either side. Gill filaments are free at the tip like in bony fish. 
• In front of the dorsal fin there is an erectile spine which in some forms is poisonous. The tail is thin and long, almost rat tail-like. 
• The mouth appears as a small opening between the lips. The upper jaw is fused with the skull and the type of jaw suspension is known as holostyly as against the hyostylic jaw suspension seen in elasmobranchs. For this reason this group is known as Holocephali. 
• The teeth appear as broad plates firmly attached to the jaws. These are bottom dwelling fish feeding on molluscs, echinoderms, crustaceans and fish which is a diverse diet considering they have a specialized dentition.
• The esophagus leads directly into a broad spiral intestine, the stomach being absent. The intestine in turn leads into a rectum. 
• Besides the pelvic claspers, as in elasmobranchs, male Holocephali have extra claspers in front of pelvic ones and an additional clasper in the head region – the cephalic clasper. 
• The notochord does not show constrictions and vertebrae are reduced to separate nodules. The open lateral line canals are found more on the underside of the snout, probably to detect food. 
• They lay large eggs considering their size; fertilization is internal. The eggs are enclosed in brown horny capsules that could be elliptical, spindle shaped Chimaera , known as ratfish, dates back to the Cretaceous period of the Mesozoic era. But many extinct Holocephali were much older and could be traced to the Carboniferous period. 

3.0Difference between Osteichthyes and Chondrichthyes