Fish (Encyclopedia of Science)
Ocean saltwater covers more than three-quarters of Earth's surface; lakes, rivers, streams, ponds, canals, swamps, marshes, and other forms of freshwater cover vast expanses of the planet's surface as well. One of the most successful groups of animals that have evolved to fill all these habitats are fish.
There are two types of fish: a small group with skeletons made of cartilage (a bonelike elastic tissue less rigid than true bone) and an enormous group with skeletons made of bone (like that found in humans). Cartilaginous fish include the sharks, skates, rays, and dogfish. The remainderore than 25,000 species (more than all other species of vertebrates combined)re known as bony fish.
All fish are cold-blooded, meaning they do not have a constant body temperature but take on the temperature of the surrounding water. The majority of fish species have bodies that are streamlined; their bodies are covered with tiny, smooth scales that offer no resistance to a fish's movement through water. The scales themselves are covered with a slimy coating that further reduces friction. Additionally, a fish's external appendages (fins) have been reduced to produce minimal resistance to the water as they propel the fish through it. Fins fall into two categories: vertical fins, which occur individually, and paired fins. Examples of the vertical fins are a dorsal fin that runs down the middle...
(The entire section is 808 words.)
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