Fish spend their lives in the comparatively dense medium of water. Their bodies are adapted to move efficiently through it and to extract oxygen from it. Fish do not have appendages but they have fins for steering and positioning them in water. Their tails are responsible for propelling them, and they have modified skeletal features of the head and throat to help them feed and breathe. There are two classes of fish based on their skeletons. Cartilaginous fish--or Class Chondrichthyes--include sharks, rays and chimeras. Bony fish with calcified skeletons are in the Class Osteichthyes..
Shark skulls have 10 cartilaginous pieces and bony fish have 63 bones in their heads. More than 30 of the bones in the skull of a bony fish are movable and controlled by more than 50 muscles. Fish don't chew their food, since this would impede the flow of water over the gills that is necessary for supplying oxygen. The various moving parts of the head skeleton open the jaws and create a feeding suction by expanding and contracting the orobranchial chamber (mouth plus gill cavities) that brings food into the fish's mouth and directs it into the intestinal system. Some bony fish have pharyngeal teeth that grind food before it passes into the intestinal tract. Sharks have upper jaws that are not attached to the rest of the skull and which can move independently.
A large bony plate under each eye is called the suspensorium. The suspensoria serve as the sides of the orobranchial chamber and provide a framework to which other elements are attached. As the suspensoria expand and contract, the volume of water the orobranchial chamber holds varies and that helps create the feeding suction. There are associated dermal bones above the suspensoria. On the floor of the orobranchial chamber are the brachiostegal rays, which serve to seal the ventral area. The hyoid bars connect the brachiostegal rays to each other and also to the suspensorium. They play a vital role in creating the feeding suction action.
Behind and attached to the suspensoria are several large bones, called opercles, subopercles and interopercles, which together form a gill plate or gill cover. When the opercles are open, water is allowed to flow out of the orobranchial chamber as it is expanded, taking water over the gills so blood can be reoxygenated. Sharks lack a gill covering.
The spine is composed of numerous vertebrae, each having downward-pointing pleural ribs anteriorly and haemal spines posteriorly, and upward-pointing neural spines. The unpaired dorsal, anal and caudal fins are supported by bony pterygiopores, which extend into the body of the fish between neural or haemal spines. The terminal vertebrae are fused into a complex structure that supports the tail fin. Sharks do not have pleural ribs.
Bones can form along the vertebral column and pleural ribs. Intermuscular bones are long and ray-like, near the fish's skin and not connected to other bones. These vary in number and location depending on the kind of fish. They consist of ossifications of the myosepta, or membranes between myomeres (lateral muscle segments).
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