Sharks are not mammals. Rather, they are a species of fish because they have a skeleton that is entirely composed of cartilage and is equipped with tough, leathery, scale less skin. These skeletons differ greatly from those of the bony fish or other terrestrial vertebrates. Cartilage is flexible, durable and has about half the amount of density as bones. As a result this helps to reduce the skeleton’s weight and conserves energy. Sharks also lack a rib cage which would result in its own weight crushing it if it were placed on land. Sharks do not have lungs that breathe air; they breathe oxygen by passing water over their gills. These creatures have highly streamlined bodies.
The earliest known shark dates as far back as 420 million years ago. Since that time, sharks have diversified into 440 different species ranging in size and length. A shark’s teeth are embedded in the gums rather than directly affixed to the jaw and are constantly replaced throughout its life. The jaw’s surface and its gill arches need extra support due to its heavy exposure to physical stress and its need for strength.
In sharks, the respiration and circulation process takes place when deoxygenated blood travels to the creature’s two chambered heart and then pumps blood to the gills via the ventral aorta artery where it branches into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries which come together to form the dorsal aorta. The blow flows from this aorta throughout the body. The deoxygenated blood then flows through the posterior cardinal veins to enter the posterior cardinal sinuses. It is here that the blood then enters the heart ventricle then causing the cycle to repeat.
Unlike mammals which are warm blooded creatures, most sharks are cold blooded meaning that their internal body temperature matches that of their ambient environment. Another thing that makes sharks differ from other mammalian species is the way in which they digest their food. A shark has an extremely short intestine. This short length of the intestine is achieved by the spiral valve with multiple turns within a single short section instead of a long tube like intestine that many mammals possess. The valves provide a long surface area that requires food to circulate inside the shark’s short gut until it has been fully digested, when remaining waste products pass into the cloaca.
Most sharks are ovoviviparous, meaning that their eggs hatch in the oviduct within the mother’s body and that the egg’s yolk and fluids secreted by glands in the walls of the oviduct help nourish the embryos. The young continue to be nourished this way until they hatch. Differing from sharks, mammals give birth to live young and feed them with milk from their mammary glands. There have also been two cases where sharks have reproduced asexually; another difference compared to mammals as mammals are now the only major group of vertebrates in which asexual reproduction has not yet been observed.