Phần I của cuốn sách này cho biết ngắn gọn về cách kiểm tra được tổ chức-Phần II sẽ giúp bạn làm quen với các khái niệm cơ bản nghỉ trên TOEFL, Phần III trình bày chiến lược wuh bạn và lời khuyên cho [anh ta câu hỏi và nhiệm vụ về phần rhe-thử nghiệm IV cung cấp cho bạn một kỳ thi thực hành wirh đầy đủ dài với câu trả lời và giải thích. | 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 armorlike 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 180 I Cracking the TOEFL iBT Cracking the Reading Section 181 On the TOEFL you will see that paragraphs referred to in the questions are marked by a this will help you quickly locate the paragraphs. You may see black squares in the passage and some of the words will be shaded in gray whereas others .
