Saturday, June 1, 2019
Sharks :: Papers
INTRODUCTIONAlthough sharks belong to the class Chondrichtyes, there atomic number 18 many different types. Sharks arose about 350 million years ago and put on remained nearly unchanged for the past 70 million years and still comprise a dominant group. It is thought that sharks almost certainly evolved from placoderms, a group of raw jawed fishes. It took a long series of successful and unsuccessful mutations with fin, jaw positions etc to give us all the different designs of sharks around today. When asked to draw a shark, most quite a little would draw a shape along the lines of the whaler shark family, tigers or a mackeral shark such(prenominal) as a porbeagle. However many hoi polloi do not realize the sheer diversity in the shape of sharks, or that rays are really sharks. Seldom does such an animal inspire such a variety of emotions reflecting a mixture of fascination, awe and fear. Sharks have occasionally exacted a terrible price from humans who have trespassed on their t erritory. No advance understood than the ocean that they inhabit, these creatures should be regarded in the same way as lions, tigers, and bears as dangerous, predatory but nonetheless magnificent animals. Different Types of Sharks Living sharks are divided into eight major orders, each easily recognizable by certain external characteristics. Each order contains one or to a greater extent smaller groups, or families. In all there are 30 families of sharks and they contain the 350 or more different kinds or species of sharks. The eight major orders of sharks include the Squantiformes, Pristiophormes, Squaliformes, Hexanchiformes, Carcharhiniformes, Lamniformes, Orectolobiformes, and the Heterodotiformes. The orders have distinguishing characteristics that fit in each. The Squantiformes normally have flat bodies that are ray-like with mottled dorsal surfaces. These sharks have a short terminal mouth, which is armed with small impaling teeth. They alike have a caudal fin, which has a lower lobe that is longer than the swiftness lobe. Their pectoral fins extend forward over the ventrally directed gills. The Pristiophormes have more of an elongated snout, which is saw-like and edged with slender, needle-sharp lateral teeth. They have two dorsal fins and no anal fin. They use short transverse mouths and small cuspidate holding teeth in both jaws. Squaliformes have no anal fin as well, but their snout is not elongated, but is somewhat long. Many have almighty cutting teeth in both jaws. In some species these razor sharp teeth are in the lower jaw only and the upper teeth serve to hold the food.