For a long time, many people believed that only humans and a few animals such as dolphins and chimpanzees were truly intelligent. Today, however, scientists are paying close attention to a very different kind of creature: the octopus. An octopus has a soft body with eight arms and no bones at all. Although it looks nothing like us, it is now considered one of the smartest animals in the entire ocean.
One reason scientists are so surprised is the octopus's unusual brain. Unlike most animals, an octopus does not keep all of its nerve cells inside its head. In fact, more than half of them are spread out through its eight arms. This means that each arm can touch, taste, and even make simple decisions on its own. Scientists believe this strange design helps the octopus react very quickly while it is hunting or hiding from danger.
Octopuses are also famous for solving problems. In experiments, they have learned to open jars in order to reach the food inside, and some have escaped from their tanks by pushing off the lid. A few octopuses even seem to recognize individual human keepers, behaving differently toward people they like and people they do not. Such behavior strongly suggests that octopuses can remember past events and learn from their experiences.
Perhaps the most amazing skill of the octopus is its ability to change its appearance. Using special cells in its skin, it can change both its color and the shape of its skin in less than a second. It does this to hide from enemies among rocks and plants, or to send messages to other octopuses. No other animal can change the way it looks so quickly and so completely.
Despite all these discoveries, much about the octopus still remains a mystery. Octopuses usually live alone and only for a few years, which makes them quite difficult to study. Yet the more scientists learn, the more they realize just how clever these strange, eight-armed animals really are. Some researchers even believe that the octopus shows us a completely different way for high intelligence to develop in the natural world.
（1） 正解 2. More than half of its nerve cells are in its arms.
第2段落に「神経細胞の半分以上が腕に広がっている」とある。選択肢2。
（2） 正解 2. They have opened jars and escaped from their tanks.
第3段落に「びんのふたを開け、水槽から逃げ出した」とある。選択肢2。
（3） 正解 1. They can remember and learn from experience.
第3段落に「経験を覚え学べることを示す」とある。選択肢1。
（4） 正解 2. By changing its color and skin shape quickly.
第4段落に「色と皮膚の形をすばやく変えて敵から隠れる」とある。選択肢2。
（5） 正解 1. They live alone and only for a few years.
第5段落に「一頭で数年しか生きないので研究が難しい」とある。選択肢1。
release：放出する
to let something out（外に出すこと）
shade：日陰
an area out of direct sunlight（直射日光が当たらない場所）
resource：資源
something useful such as water or energy（水やエネルギーなど役に立つもの）
charity：慈善団体
a group that helps people in need（困っている人を助ける団体）
confirm：確認する
to say that something is definitely true or correct（間違いないと知らせる）
nerve cell：神経細胞
a cell that carries signals in the body（体内で信号を運ぶ細胞）
recognize：見分ける
to know someone you have seen before（以前に見た相手だとわかる）
appearance：見た目
the way someone or something looks（人や物の見え方）