Like all vertebrates(脊椎动物), fish have sense organs that tell them what is happening within

Like all vertebrates(脊椎动物), fish have sense organs that tell them what is happening within their environment. The organs enable them to see, hear, smell, taste and touch.
A fish’s eyes differ from those of land vertebrates in several ways. For example, most fish can see to the right and to the left at the same time for the fact that a fish has no neck and so cannot turn its head. Fish also have no eyelids. For land vertebrates, eyelids(眼睑）help moisten(使湿润) their eyes and protect them from sunlight. A fish’s eyes are kept moist by the flow of water over them. They do not need to be shielded from sunlight because it is seldom extremely direct and bright underwater. Some fish have unusual adaptations of the eye. For example, a flatfish has two eyes on the same side of the head. A flatfish spends most of the time lying on its side on the ocean floor and so needs eyes only on the side that faces upward. The eyes of certain deep-sea fish are at the ends of short structures that stick out from the head. These structures can be raised upward, allowing the fish to see overhead as well as to the side and to the front.
A few kinds of fishes are born blind. They include certain species of catfish that live in total darkness in the waters of caves. Some of these fishes have eyes but no vision(视觉). Others lack eyes pletely.
【小题1】Why can’t a fish turn its head? (no more than 6 words)
    
【小题2】What can prevent land vertebrates’ eyes from sunlight? (no more than 3 words)
    
【小题3】Why are the flatfish’s eyes different from other fishes’? (no more than 14 words)
    
【小题4】What is the passage mainly about? (no more than 6 words)
    Because it has no neck.