Characterizing sharks: Sharks are jawed fishes. Like all fishes they are aquatic, water-breathing vertebrates with a brain and spinal cord; fins; plate-like pairs of internal gills; and paired sense organs. Unlike most bony fishes, sharks do not have swim bladders or lungs. Sharks' jaws are simple but effective structures, armed with transverse rows of hard teeth that are replaced slowly but continuously. These teeth are not in sockets, but attached to the jaws by soft tissue.
Fins: All sharks have paired fins - pectoral and pelvic - on the underside of the body. The pectoral, or breast, fins are located just behind the gill region of the head. The pelvic fins are on the rear of the abdomen just in front of the tail. In male sharks, each pelvic fin has a clasper, or copulatory organ, for implanting sperm inside the vent (the common reproductive and excretory opening) of the female. The vent is located between the pelvic fins in both sexes. Most sharks have a cylindrical or slightly depressed head and body; a strong tail with caudal, or tail, fin; one or two dorsal fins on their back; and an anal fin on the underside of the tail behind the vent.
The skeleton: The skeletons of cartilaginous fishes are composed of cartilage instead of bone, similar to the material found in the human nose and ears. The prominent scales, bones of the head and bony fin rays of bony fishes are absent in sharks. Instead, their bodies are covered with small, tooth-like scales, often referred to as denticles.
Inside a shark: A shark's body includes two cavities: the pericardial cavity, jsut below the gills, which contains the heart; and the trunk, or visceral cavity, which begins below the esophagus and contains other internal organs. The head and truck enclose a tubular gut, which includes the digestive system. This tubular gut begins in the head at the mouth and ends at the vent, or excretory opening, between the pelvic fins. The mouth opens into the pharynx, which is connected, via the esophagus to the stomach.
The visceral cavity: The stomach often extends through part or all of the visceral cavity and then doubles back, connecting to the first part of the intestine, or duodenum, by way of a tubular section known as the pylorus. This attaches to the valvular intestine, which in turn extends to the rectum. The rectum discharges solid waste into the cloaca and this waste is expelled through the vent. The visceral cavity also contains a central gall bladder and the liver, which is often very large and extends rearward to fill the lower part of the visceral cavity.
Sharks provide a host of benefits. Their meat is eaten or used as fertilizer, and their fins are made into soup. Oil rich in Vitamin A is extracted from their livers, and anticoagulants from their blood. Their eyes provide corneas for transplants, and their cartilage has several medical applications.
Sharks as food: In the past, shark flesh was regarded by many western consumers as inferior and even inedible, and was often marketed under alternative names, such as "rock salmon." Recently, however, shark meat (especially thresher and mako) has gained in popularity. Shark fin soup has been a Chinese delicacy for centuries, and the modern demand for shark fins has made them an extrememly valuable commodity. Greek and Roman gourmets ate shark with enthusiasm, and it remains a staple of Mediterranean food as well as that of Asia and the Pacific.
