题目

Starfish(海星)are sea animals. Starfish are also called sea stars. This is because of their shape. They look like stars. There is a word “fish” in their name, but they are not fish. They are echinoderms(棘皮动物). Starfish have different sizes. Some starfish are very small. They are only one inch (英寸)wide. Some starfish are very large. They are three feet wide. Starfish not only have different sizes, but also have different rays. Most sea stars have only five rays or arms. Some, can have as many as twenty! Sea stars come in many different colors. Some even have special pat19．体育室里有18个篮球和15个皮球．我借9个皮球，体育室还剩几个皮球？