There are many different varieties of fish from goldfish to sharks. Fish do have a lot of the same characteristics as people such as eyes, mouth, skeleton, muscles, heart, liver, blood vessels, brain, and kidneys. They also have many things people do not such as gills, fins, and a bladder. All fish have a spine but there are two main kinds of fish,cartilaginous fish and bony fish. This just means that the skeleton of the bony fish are indeed made of bone, while the cartilaginous fishes have a cartilaginous skeleton, such as sharks and dogfish. The more common of the two is the bony fish.
Fish breathe by absorbing the oxygen dissolved in the water around them through their gills. They also have scales covering the outside of the body, bony fish have flattened scales covered with a thin skin and a mucus like substance that cuts the friction between the fish and the water, giving them that slippery feel. The cartilaginous fishes' scales are spiky and protrude through the skin, making their scales feel more like sandpaper.
The external features of the fish include the nostrils, which they do not use for breathing, but for smelling. Their nostrils are very sensitive. As most people know, they eyes of the fish are on either side of the head and have large pupils. The fish does not necessarily have ears, but they do hear. They hear by vibration through the sensitive inner ear. The mouth serves to take in food. The lateral line in the fish is a jelly filled canal below the skin and opens to the water by way of a small set of pores. The canal has many nerve endings that detect movement in the water causing the jelly substance to vibrate and warning the fish to danger.
The fins are likely the most noticeable part of the fish, and there are many different kinds of fins. The fins are used mainly for stability and direction control. The tail fin helps to propel and guide the fish while the median fins, including include the dorsal, anal and ventral fins, control the rolling of the fish . The pectoral and the pelvic fins control the upward and downward movements of the fish.
One of the very unique features to the fish is the swim bladder and only bony fish have this. A shark or a dogfish will sink if they stop swimming, while the fish with the swim bladder will be able to float. The bladder runs just below the spine and is filled with air. In some fish the bladder opens to the gut area, making it possible for the air pressure to be increased or decreased by letting air into the mouth or releasing it. It is an extraordinary feature to these creatures. Also, fish are most often referred to as “cold-blooded” when indeed they are not, they adapt to the water temperature around them, so therefore if the water is cold, then so shall the fish be, but the same goes for warm waters.