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Sharks and rays have very large and complex brains, but little is known about their brains.
A sharks brain to body mass ratio is higher than most other fish and is comparable to many other vertebrates, including some mammals.
Brain size and complexity vary from shark species to species. The sharks with the largest brain to body weight ratio are active sharks like the dusky shark and the scalloped
hammerheads. The sluggish bottom-dwellers, like angelsharks, have relatively smaller, less complex brains.
Eyes are lateral on sharks, dorsal on batoids (manta and sting rays).
Some species have an eyelid like structure called a nictitating membrane. The nictitating membrane protects the eye from being injured by thrashing prey while the shark is feeding.
Eye size and position vary, depending on the particular habitat or behavior of the species. In general, deep water sharks have bigger eyes than shallow water sharks.
Sharks and batoids have ventral and external nostrils. Some species have nasal barbels, sensory projections near the nostrils and mouth, like those found on nurse sharks.
MOUTH AND TEETH:
On both sharks and batoids, the mouth is usually ventral. It is located at the tip of the snout in the whale shark, megamouth shark, frilled sharks, and some carpet sharks.
The mouth may have labial folds or furrows.
Teeth are modified, enlarged placoids scales. Sharks may have up to 3,000 teeth at one time in numerous rows attached at their bases to connective tissue. Several rows of replacement teeth continually develop behind the outer row of functional teeth. As the old teeth fall out, new teeth continually take their place.
Most sharks do not chew their food, but gulp it down whole it in large pieces. Their
teeth are not able to chew food, but only to tear it into mouth-sized pieces. Some
bottom dwellers crush the shell of their prey, but still do not thoroughly chew the food
like we do.
A shark's teeth are the only part of its body to fossilize since they are composed of bone. Many fossilized shark teeth have been found, including that of megalodon. Its teeth resemble those of the great white shark but are almost 3 times larger.
Shark teeth are made the same as the denticles in the shark's skin and also the same as human teeth. There is a pulp cavity in the center, covered by dentine and this has a hard enamel on the outside.
The Mechanics of a Goblin Shark's Jaw...
A shark's tongue is called a basihyal. A shark's tongue is a small, thick, relatively immovable piece of cartilage that is found on the floor of the mouth of sharks and fishes. The tongue seems to be useless for most sharks, except for the cookiecutter sharks, who use it, along with very sharp teeth and suction, to cut cookie-shaped bites out of their prey. The taste buds of sharks are located in their mouths, not in their tongues.