Earth Science: Biology – Animals
Animals are multicellular organisms that—in contrast to plants—must take in nourishment in the form of organic substances. These substances are generally broken down and absorbed in special cavities within the body (digestive systems). Animals can be divided into two large groups: invertebrates and vertebrates. The first group, composed of animals without a vertebral column, are sometimes referred to as lower animals.
They include by far the largest number of species, even the highly diverse group of insects, along with others such as worms, bivalves, and gastropods. The vertebrates, on the other hand, include more highly developed animals, from fish to amphibians, reptiles, birds, and mammals, and thus also human beings.