填空题

To us, the environment in which fish dwell often seems cold, dark and (36) .But there are advantages to (37) water. And (38) played an important role in making fish what they are. One is that water isn’t (39) sudden temperature changes. Therefore it makes (40) habitat for a cold-blooded animal. Another advantage is the water’ s ability to easily support body weight. Protoplasm has (41) the same density as water. So fish in water is almost weightless. This weightlessness in turn means two things: 1) Fish can get along with a light weight and a simple bone (42) . And 2) Limitations to a (43) size are practically removed. Yet there is one basic difficulty to live in water in fact that it is incompressible. (44) The fish pushes water aside by the forward motion of its head and with a curve of its body and its flexible tail. (45) . The fact that water is incompressible has literally shaped the development of fish. A flat and angular shape can be moved through water only with difficulty. (46) .