Our multimillion nerve-cell central nervous system has its roots in the scattered nerve cells of tiny, lowly organisms that lived in water half a billion years ago. Nerve cells evidently first appeared in coelenterates — “hollow-gutted” organisms like the hydra and the sea anemone. A coelenterate’s nerve network lacks any kind of centralized control. This probably began with flatworms -- the first creature to possess a head.
Specialized sense cells help flatworms respond more flexibly than sea anemones to outside stimuli. But like most animals without a backbone, flatworms act almost by instinct and reflex.
Intelligent behavior remained impossible until the appearance of relatively big, complex types of brain — the types we find among the backboned animas or vertebrates. The tiniest fish has a larger brain than the largest insect. But the development of a fish's three-part brain reflects that beast’s unintellectual priorities. Much of the forebrain deals only with smell. The midbrain handles vision and the hindbrain balance.
With early mammals the brain grew larger and more complex. Sense coordination shifted from the midbrain to the forebrain, a developing structure capped by folded cerebrum to handle memory and learning. Meanwhile the hindbrain gained a large cerebellum to coordinate complicated movements.
Advanced mammals such as monkeys, apes and human (the primates) have brains derived from ancestors that took to living in the trees, where vision mattered more than smell. Accordingly the once big “smell’’ part of the forebrain grew smaller, while the part that handles vision grew much larger.
1.With what topic is the passage mainly concerned?
2.The hydra is a kind of（ ）.
3.In the last paragraph, the phrase “took to” could best be replaced by which of the following?
4.It can be inferred from the passage that insects do not have（ ）.
5.According to the passage, what helps to coordinate the complex physical activity of a mammal?
