Sharks and the dogfish Mayfield City Schools（鲨鱼和角鲨梅菲尔德城市学校）.pdfVIP

Sharks and the dogfish Biologically, sharks are fish belonging to the phylum Chordata and the subphylum Vertebrata. However, sharks and their relatives, the rays and skates, are unique amongst fish in that their skeletons are made entirely of cartilage, not bone. This places them in the class Chondrichthyes, subclass Elasmobranchii and the order Selachii. The bony fish, the Osteichthyes, possess a gas-filled swim bladder by means of which they can regulate their buoyancy allowing the fish to float at various depths under water. Sharks have no swim bladders. They are somewhat heavier than the water they displace. Thus, once a shark ceases to move, it sinks. Coastal species rest on the sea floor in shallow water. However, the sharks of the deeper oceans must continue moving from the moment of birth to the moment of death. If they were to stop swimming, they would sink and be crushed by the pressure of the deep below. Regulation of osmotic pressure in marine sharks differs from that of their bony relatives. They retain a high concentration of urea and other solutes in their body fluids, and a concentration of salts higher than that in the surrounding sea water. Therefore, sharks do not need to drink water as the hypotonic ocean water moves INTO their tissues in an attempt to dilute the more concentrated body fluids. The spiny dogfish, genus and species name Squalus acanthias, of the family Squalidae, is our dissection specimen. The species name acanthias calls attention to the animals mildly poisonous spines, one in front of each dorsal fin. The absence of an anal fin is characteristic of the entire family. Fertilization is internal, and most shark pups hatch internally, to continue their development within the uterus of the mother. After a period of gestation (up to two years in the spiny dogfish, which is longest of any vertebrate) they are born alive as a smaller version of the adult. This method o