Sunday, December 21, 2008

Sharks

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics. They have several sets of replaceable teeth. Sharks range in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres and which feeds only on plankton, squid, and small fish through filter feeding.

Shark
Photo: theage.com.au

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. It is a fish. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks. The first sharks appeared in the oceans 350 to 400 million years ago. Most of the species we know today are as old as the Jurassic period.

The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off 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 blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats.

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