2019屆高三英語Under the sea
shell
in addition to their reptilian traits, all species of turtles except the leatherback have evolved a bony outer shell, which protects them from predators. the leatherback is the only soft-shelled sea turtle and is in a family by itself. the shell covers the dorsal (back) and ventral (belly) surfaces of a sea turtle. the dorsal portion of the shell is the carapace and is covered with scale-like structures called scutes, while the ventral portion of the shell is known as the plastron. experts can identify species of sea turtles by the number and pattern of scutes on the carapace. the shell is considered the most highly developed protective armor of any vertebrate species. while most land turtles can retract their heads into their shells for added protection, sea turtles cannot and their heads remain out at all times.
color
green sea turtles have green flesh. the carapace is olive brown to black, while the plastron is pale yellow to creamy white. the carapace is molted, variegated in color. rarely will you find a green sea turtle with a solid colored shell. counter-shading of the shell conceals the turtle from predators, making it difficult to distinguish the dark carapace from the sea floor and the light plastron from the lighter sky.
size
green sea turtles as hatchlings weigh about an ounce and have a carapace length of 2 inches. a sexually mature green sea turtle weighs 200-350 pounds with a carapace length of 2.5 feet. adults grow to a carapace length of 3.5 feet and weigh an average of 400 pounds. one of the largest turtles that ever lived from the late cretaceaus period, 144 to 65 million years ago, reached a length of 9.8 -13 feet.
lifespan
the lifespan of sea turtles is not known. it is believed that green sea turtles reach sexual maturity around the age of 25 years and can live up to 80 years of age. the long period of maturation helps to explain why it takes turtles so many years to recover from a population decline.
shape
sea turtles are wonderfully adapted to life in the ocean. their shells are lighter and more streamlined than land turtles. front and rear limbs have evolved into flippers. these flippers make sea turtles efficient and graceful swimmers, capable of swimming long distances in a short time.