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 offish. A flat and angular shape can be moved through water only with difficulty. (46)____________.