请用英文介绍海豚是什么动物

微软百科上的一篇很全面的介绍：

Dolphin (aquatic mammal)
I INTRODUCTION

Dolphin (aquatic mammal), fast-swimming animal related to whales and porpoises. Sleek and powerful swimmers found in all seas, dolphins are distinguished from porpoises by well-defined, beaklike snouts and conical teeth. The porpoise has a blunt snout, chisel-shaped teeth, and a stouter body.

There are at least 32 species of dolphins. Typical examples are the bottle-nosed dolphin, a popular performer in seaquariums, and the common dolphin, which inspired many Mediterranean folk legends. Both often appear in open waters, making their characteristic arched bounds, frequently before the bow waves of ships. Several freshwater species inhabit river estuaries in Asia and South America. The small, graceful tucuxi dolphin has been sighted more than 2000 km (more than 1250 mi) up the Amazon River. The tucuxi, the smallest dolphin, is less than 1.2 m (less than 4 ft) long; the largest, the bottle