Sharks are ferocious predators in the sea that are equipped with a full cartilaginous skeleton. They accomplish respiration through a series of five to seven gills, which are respiratory organs of most aquatic animals that breathe water to obtain oxygen, consisting of a filamentous structure of vascular membranes across which dissolved gases are exchanged. A unique characteristic that sharks possess is their teeth that are replaceable, and very numerous. As shown in the picture below, a shark may lose thousands of teeth in its lifetime.
Sharks range greatly in size. They range from the small Etmopterus perryi to the whale shark. There are advantages and disadvantages to both sizes. Some larger species do not physically have the capability to support their own body weight, but smaller sharks have to worry about those who are hunting them as prey.
The skeleton of a shark is composed mostly of cartilage. The body shape of the shark makes it possible to easily maneuver the water around its prey. A key element of the shark's structure is the fin which help navigate the shark through the water with agility and speed.
The caudal fins of the shark vary between species because of the different lifestyles of the shark. The fins are used to propel the shark through the water with speed as well as steer the organism. The fins are usually adaptive to the conditions and environment of the shark.
Like most other animals, sharks reproduce sexually. The mating rituals that occur sometimes become very violent and may sometimes involve things such as synchronized swimming, biting and color changes. Many sharks may be seen with scars which had been caused by the mating rituals which had been carried out. Once the male and female has mated the female is internally fertilized. This begins the embryo developing process. Depending on the species, there are three ways of developing the embryo. These three ways are Oviparous sharks, Viviparous sharks, and Ovoviviparous sharks. Oviparous sharks lay "thick cases that are resistant to predators, which they attach to rocks or seaweed. The eggs hatch days, or weeks, later, leaving the young to fend for themselves". Viviparous sharks "give birth to live young, which are nourished in the female shark’s uterus via a placenta, or a secretion known as uterine milk. Viviparity ensures that the young are very well nourished during development and thus fit to survive the rigours of the sea immediately after birth". "Ovoviviparous sharks also support the embryos internally and give birth to live young, but they do not provide any direct nourishment to their offspring. Instead, the developing sharks rely on the egg’s yolk sac for sustenance."
In sharks, respiration is accomplished through a set of gills which are respiratory organs for most aquatic animals that breathe water to obtain oxygen, consisting of a filamentous structure of vascular membranes across which dissolved gases are exchanged. Ram ventilation is a term referring to the ability to keep adequate oxygen levels in the lungs. Sharks are physically unable to stop swimming because moving and swimming throughout the water is how the shark acquires oxygen but if it stops they oxygen level is to low and the shark suffocates. De-oxygenated Blood traveling to the two-chambered heart begins the respiration process. Blood then flows to the gills through the sharks ventral aorta artery where the blood becomes re-oxygenated. The blood then flows from the dorsal aorta throughout the body to all of the muscles and veins. The sharks respiration is not very effective due to the inability to stay still because of the lack of oxygen.
Sharks are able to float because of their unique large liver filled with oil that contains squalene. Because of this liver, some shark species when flipped over enter a state of inactivity and immobility.
Unlike most animals, sharks teeth are embedded into their flesh, and not in the jaw. All shark species have a series of teeth both on the upper and lower jaw, which are constantly changed or replaced by the many teeth that are in rows behind them. A sharks teeth usually are changed about every 8-10 days. The lower teeth of a shark are primarily used for the holding of prey, while the upper teeth are used for cutting or biting the 
Smell: The sense of smell in sharks is carried out by drawing water onto the olfactory sacs. The shark is then able to determine the various smells in the water. They are also able to detect electric fields in the water which are emitted by their prey. Sharks are drawn to the smell of blood and other chemicals which are put off by the guts of their prey.
Vision: Sharks eyes themselves are very similar to human eyes. Some of the similar characteristics are that we both have a cornea, an iris, a pupil, a lens and a retina. In good conditions, sharks are able to see up to 50-60 feet under what because there sight is up to 7x better than humans. A tapetum lucidum is used to increase the sensitivity of the sharks eyes to the light in the environment.
Hearing: Sharks have an acute sense of hearing, so much so that they are able to hear their prey from miles away through small opening on each side of their heads that leads straight to the inner ear. Lateral line pores distinguish vibrations around the environment.
Sharks are known as hunters in the ocean realm. Although have a history of attacking humans, they do not attack unless they feel threatened or mistake a human being as prey. Most sharks move in groups of up to 100.
Evolution and Sharks