The most plenty plants in the ocean are known as phytoplankton(浮游生物)．These are usually single－celled, minute floating plants that drift throughout the surface waters of the ocean.To grow, phytoplankton need nutrients from the sea water and lots of sunlight.

　　Marine animals are divided into three groups：zooplankton, nekton, and benthos.Zooplankton are drifting animals and are usually small, however, they can grow to fairly large size.For example, the jellyfish and the Portuguese man－of－war are examples of larger types of zooplankton which are unable to propel(推进) themselves effectively.

　　Nekton are the free swimmers and probably the largest portion of familiar animals found in the ocean belong to this class.Common fishes, the octopus, whales, eels and squid are all examples of nekton.

　　The third type of sea animals spends its entire life on or in the ocean bottom.This group of marine animals is called the benthos.Some of these creatures, such as lobsters and snails, may be able to move about on the bottom but their lifestyle is so bound up with the ocean floor that they are unable to survive away from this environment.

　　【语篇解读】　海洋是成千上万种动物和植物的家乡。海洋植物主要有两种：有根植物和浮游植物；海洋动物大体可分为三类：浮游动物、游泳动物和海底动物。

　　21．The text mainly tells us something about ________．

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