The Animals

Animals are multicellular heterotrophs whose cells lack cell walls and that have a motile stage at some part of their life cycle. They are diploid and reproduce primarily by sexual reproduction. Some of the interactions between animals and the environmental effects of human activities are:

• Loss of wildlife habitat may cause extinction.

• Overharvesting may lead to depletion or extinction of species.

• Animals may be reservoirs for human diseases.

• Control of pests may cause pollution.

The animal kingdom is divided into 33 phyla. A few of them include familiar groups or organisms. However, quite a few are relatively unfamiliar, and most of these are small marine organisms. About 1.5 million animal species have been named, but these are probably only a fraction of those that exist. Table 8.1 summarizes the animal phyla, grouped according to an informal subdivision system.

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