Sharks and Mantas
Sharks have mouths on their undersides and water passes through the mouth and over the gills and out through the slits. With bottom-dwelling mantas and skates this would cause mud to get into the gills, so instead, they have two openings or spiracles on the upper surface of the head that take in water and lead it straight to the gills. It is then expelled on the underside through the gills. One kind of ray, the manta has reverted from bottom-dwelling to surface dwelling, using the large lateral extensions to remain afloat.
Sharks are a group (superorderSelachimorpha) of fish, with a full cartilaginousskeleton, a streamlined body plan, with normally 5, but up to 7 (depending on species) gill slits along the side of, or beginning slightly behind, the head (in some species, a modified slit called a spiracle, is located just behind the eye), dermal denticles covering the body to protect from damage, parasites and improve fluid dynamics, and rows of replaceable teeth in the mouth.
HUMANS TASTE BAD
Contrary to popular belief, only a few sharks are dangerous to humans. Out of the more than 360 described species of sharks, only 4 have been confirmed to have killed large numbers of humans: the Great White, Tiger, Bull, and Oceanic Whitetip sharks. These sharks, being large, powerful predators perfectly capable of eating humans, will sometimes attack and kill people, but all of the above sharks, even the Great White, have been filmed in open water, with no cage, time and time again, without incident. There are many theories about why sharks attack people. Some claim that the shark is confusing a human for a seal or other prey animal; this would be typical in the case of an attack against a surfer. Often the shark that attacks a human will make only one bite and then go away. This behaviour has many possible explanations, one being that humans don't taste good (or at least, as good), or are lacking the necessary fat, and another being that sharks normally make one swift attack, and then retreat and wait for the victim to die, or exhaust itself, before it comes back to feed. This protects the shark from retribution from a wounded and aggressive target, but also allows humans the time to get out of the water and survive.