English for

5.4 Reading Resource #1
: Introduction 
While we can easily identify dogs, lizards, fish, spiders, and worms as animals, other animals, 
such as corals and sponges, might be easily mistaken as plants or some other form of life. Yet 
scientists have recognized a set of common characteristics shared by all animals, including sponges, 
jellyfish, sea urchins, and humans. The kingdom Animalia is a group of multicellular Eukarya. Over 
600 million years ago, tiny creatures that do not resemble any living organism today began animal 

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evolution in the ocean. Animals have evolved into a diverse kingdom since then. Although over one 
million currently living species of animals have been identified, scientists are continually discovering 
more species. The number of known living animal species is estimated to be around 1.4 million, but 
it could be as high as 6.8 million. 
Features of the Animal Kingdom 
Despite their incredible diversity, members of the animal kingdom share common 
characteristics that set them apart from organisms in other kingdoms. Animals are all eukaryotic, 
multicellular organisms, and nearly all have specialized tissues. Most animals are motile, at least 
during certain life stages. Animals require food to grow and develop. All animals are heterotrophic, 
meaning they consume both living and dead organic matter. This mode of energy acquisition 
distinguishes them from autotrophic organisms, such as most plants, which produce their own 
nutrients via photosynthesis, and fungi, which digest their food externally. Carnivores, herbivores, 
omnivores, and parasites are all types of animals (Figure 5.18). Most animals reproduce sexually, and 
their offspring go through a series of developmental stages that establish a specific body plan, unlike 
plants, which have an indeterminate body shape. The