To us, the environment in which fish dwell often seems cold, dark and【B1】___

鱼在水中运动的原理
    To us, the environment in which fish dwell often seems cold, dark and (36) mysterious. But there are advantages to (37) live in water. And (38) they’ye played an important role in making fish what they are. One is that water isn’t (39) subject to sudden temperature changes. These fore it makes (40) an excellent habitat for a cold-blooded animal. Another advantage is the water’s ability to easily support body weight. Protoplasm has (41) approximately the same density as water. So fish in water are almost weightless. This weightlessness in turn means two things: 1) Fish can get along with a light weight and a simple bone (42) structure. And 2) Limitations to a (43) fish’s size are practically removed. Yet there is one basic difficulty to live in water-- the fact that it is incompressible. (44) For a fish to move through water, it must actually shove it aside. Most can do this by wiggling back and forth in a snakelike motion. The fish pushes water aside by the forward motion of its head and with a curve of its body and its flexible tail.
   (45) Next the water flows back along the fish’s narrowing side, closing in at the tail and helping the fish propel itself forward. The fact that water is incompressible has literally shaped the development of fish. A fiat and angular shape can be moved through water only with difficulty. (46) And for this reason, fish have a basic shape that is beautifully adapted to deal with this peculiarity.