43).There were also crablike creatures, whose bodies were covered with a horny substance.The body segments each had two pairs of legs, one pair for walking on the sandy bottom, the other for swimming.The head was a kind of shield with a pair of compound eyes, often with thousands of lenses.They were usually an inch or two long but some were 2 feet.
44). Of these,the ammonites are very interesting and important. They have a shell composed of many chambers, each representing a temporary home of the animal. As the young grew larger it grew a new chamber and sealed off the previous one. Thousands of these can be seen in the rocks on the Dorset Coast.
45).About 75 million years ago the Age of Reptiles was over and most of the groups died out. The mammals quickly developed, and we can trace the evolution of many familiar animals such as the elephant and horse. Many of the later mammals though now extinct, were known to primitive man and were featured by him in cave paintings and on bone carvings.
[A]The shellfish have a long history in the rock and many different kinds are known.
[B]Nevertheless,we know a great deal about many of them because their bones and shells have been preserved in the rocks as fossils. From them we can tell their size and shape, how they walked, the kind of food they ate.
[C]The first animals with true backbones were the fishes,first known in the rocks of 375 million years ago. About 300 million years ago the amphibians, the animals able to live both on land and in water, appeared. They were giant, sometimes 8 feet long, and many of them lived in the swampy pools in which our coal seam, or layer, or formed. The amphibians gave rise to the reptiles and for nearly 150 million years these were the principal forms of life on land, in the sea, and in the air.
[D]The best index fossils tend to be marine creature. There animals evolved rapidly and spread over large over large areas of the world.
[E]The earliest animals whose remains have been found were all very simple kinds and lived in the sea. Later forma are more complex,and among these are the sealilies, relations of the starfishes, which had long arms and were attached by a long stalk to the sea bed, or to rocks.
[F]When an animal dies,the body, its bones, or shell, may often be carried away by streams into lakes or the sea and there get covered up by mud. If the animal lived in the sea its body would probably sink and be covered with mud. More and more mud would fall upon it until the bones or shell become embedded and preserved.
[G]Many factors can influence how fossils are preserved in rocks. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form.
［试题分析］
本篇是一篇科普文章，介绍的是史前类动物。文章结构比较清晰，首先用一般性的介绍来引入史前类动物和记载它们形体和活动的化石，接下来的几段就逐渐按照进化的顺序来一一介绍各种动物。
具体分析一下五个空白处的内容。