听力音频：

• organisms without a backbone
• commonly are called snails or slugs (without a shell)

• the mineral composition of its body fluids is close to that of the 31. ______
• fossilized remains founded in rocks dating back 500 million ears ago

• all developed a foot for swimming or walking
• snail shells are different in form, but most of which coil to the 32. ______
• some snails have ribs, spines, and even 33. ______ on their shells

• fed on plants and animals, e.g. the shield slugs prey on 34. ______

• birds, frogs, flies and human
• brought into Britain as a food source in 35. ______
• for self protection, gastropods have evolved some unique 36. ______

37. ______ condition is the worst for most species

• most gastropods species like to live in old 38. ______
• while a few live beneath the 39. ______ (e.g. blind snails)
• indicator of the quality of 40. ______ standards

答案