TAILIEUCHUNG - Cracking the Toefl IBT 2009 edition part 22

Tham khảo tài liệu 'cracking the toefl ibt 2009 edition part 22', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Shell layer called the exocuticle. With its molecules bonded so tightly the exocuticle is very durable. There are points on the body where it does not form since flexibility is needed around joints. This arrangement allows supple movement but provides armor-like protection. Though strong the chitin and protein exocuticle itself would provide a poor barrier against moisture loss. Therefore it must be coated with lipids which are also secreted by the epidermis. These lipids mostly fatty acids and waxes form the third outermost layer of the shell. They spread over the cuticles to form a waterproof seal even in dry weather. This lipid layer gives many arthropods their distinctive luster. Combined the endocuticle exocuticle and lipid coating form a shell that provides formidable protection. The external shell has other advantages. One is that because it has far more surface area than the internal skeleton found in vertebrates it provides more points at which muscles can be attached. This increased number of muscles permits many arthropods to be stronger and more agile for their body size than birds or mammals. The coloration and markings of the exoskeleton can be beneficial as well. Many species of scorpion for instance have cuticles that contain hyaline. The hyaline is excited by ultraviolet radiation so these scorpions glow blue-green when a black light is flashed on them. Scientists are not sure why scorpions have evolved to fluoresce this way but the reason may be that their glow attracts insects that they can capture and eat. Adaptive as their shell is it leaves arthropods with at least one distinct disadvantage The cuticle cannot expand to accommodate growth. As the animal increases in size therefore it must occasionally molt. The existing cuticle separates from newer more flexible layers being secreted beneath it gradually splits open and can be shaken or slipped off. The new chitin and protein will harden and be provided with a fresh lipid coating but this .

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