These are also known as the cartilaginous fishes. They have an endoskeleton of cartilage and are cold blooded. The placoid scales form the exoskeleton and are dermal in origin. They do not have a gill cover so the gills are uncovered. Their jaws are well developed and have a ventrally placed mouth. They have a scroll valve in the intestine. The digestive tract makes the way to cloaca. They have the external nares but no internal nares are present. They have a couple of fins which are broad in the outlook. They are mainly marine and are predators. The process of excretion and respiration occurs by a pair of kidneys which are mesonephric and gills respectively. They have multiple cranial nerves. The urea is main waste product. Their heart is double chambered with well developed SV and CA. The development is mainly indirect with the formation of larva. They have an internal mode of fertilization. They have no air bladder and can be oviparous or ovoviviparous. The brain has cerebellum and well developed olfactory lobes. They have well developed lateral line system and have claspers which help in the copulation. It has a well formed renal portal system.
The chimaera is also known as rabbit fish. It occurs in the deep portion of the sea. It has gill covers. It is the connecting link between shark and bony fish. It has a skeleton made up of cartilage and has a couple of pelvic claspers. It also has a placoid scale. They have no cloaca. They have separate anus and urinogenital aperture. They have large pectoral fins. Their tail resembles a whip.
Scolidon is also known as the dog fish. It occurs in the Bay of Bengal mainly. It is a carnivorous and a surface feeder. The upper and lower jaws bear the similar teeth known as homodont teeth. They help to catch the prey and prevent its escape as they are pointed backwards. They do not help in the mastication. They do have pores which help in to gain the information about the changes in the temperature. These are known as the ampullary pores which lead into the ampulla of lorenzini. They also have gill slits which are present on the back side of head and are oblique in shape. They have a communication with the pharynx. They have a tail where the lobes of different kinds are present and are known as heterocercal tail. The placoid scales form the exoskeleton which is placed in the skin. They help in the protection. They have well developed lateral line system and the males do have claspers which help in the copulation. It is a good source of liver oil.
The torpedo is also known as an electric ray. It occurs in the deep portion of the sea and is a carnivorous. It has no placoid scale and has a smooth skin. They discharge electricity to stun the organisms. It has spiracles as the breathing organs for respiration. It has a couple of electric organs for the purpose of generating electric current. The couple of pectoral fins are fused. They do have gill slits. The trygon is also known as a sting ray. It has the electric ray without the presence of electric organs. Its body resembles a kite and tail resembles the whip.
There are a lot of differences between a shark and ray. The shark feeds on the surface and its body is spindle in shape and laterally compressed. The ray feeds on the bottom and its body is disc in shape and dorsoventrally compressed. The sharks have unique pectoral fins and spiracles may or may not be present. The ray has no unique pectoral fins and spiracles are present. There are gill slits present on the lateral side of body in shark but in ray they are present on the ventral surface of body.
