Fishes
Fishes are aquatic vertebrates; most have paired fins, scales and gills.
Most fishes exchange gases using gills locaed either side of the pharynx. The gills are made up of feathery, threadlike structures called filaments.
Fishes have closed circulatory systems with a heart that pumps blood around the body in a single loop, to the heart to the gills. The heart in made up of 1st the Sinus Venosus, then the Atrium then to the Ventricle and finally to the Bulbus Arteriousus. Bony fishes make up the Class Osteichthyes. (Bellow) The skeletons of these fishes are made of hard, calcified tissues called bone. Almost all bony fishes belong to a huge group called ray-finned fishes.
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The Class Chondrichthyes (Bellow) contains sharks, rays and sawfishes. The skelton of these fishes are built entirely of cartilage, not bone. Most of the living shark species have a large curved tails, torpedo-shaped bodies and pointed snouts. Some are filter feeders, and many are carnivorous.
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