Like all living things, animals show similarities and differences that enable them to be classified into groups. Birds, for example, are the only animals that have feathers, while mammals are the only ones that have fur.
The scientific classification of animals began in the late 18th century. At this time, animals were classified almost entirely by external features, mainly because these are easy to observe. But external features can sometimes be misleading. For example, in the past, comparison of physical features led to whales being classified as fish and some snakes being classified as worms.
Kingdom Animalia includes more than one million living species, grouped into more than 30 phyla. Vertebrates, members of the phylum Chordata, comprise only one percent of these organisms. Phylum Arthropoda is more successful in sheer numbers, total mass, and distribution than all other groups of animals combined. The remaining animal phyla are composed of mostly marine-dwelling organisms. Illustrated here is the evolutionary relationship between all of these groups.
Presently, animals are classified according to a broader range of characteristics, including their internal anatomy, patterns of development, and genetic makeup. These features provide a much more reliable guide to an animal's place in the living world. They also help to show how different species are linked through evolution. Scientists divide the animal kingdom into approximately 30 groups, each called a phylum (farm plural phyla park).
The basic body plan of an animal, shown here in cross section, is one of the characteristics used to classify animals into separate phyla. Cnidarians such as jellyfishes and sea anemones have two layers of tissue, endoderm and ectoderm, surrounding a digestive cavity. In some animals a third layer, mesoderm, develops between the endoderm and ectoderm. Among these, flatworms and ribbon worms are called acoelomates, because they lack a separate body cavity, or coelom. Nematodes have an extra, epithelial-lined cavity called a pseudocoelom, but only animals such as annelids and chordates have a true coelom, a fluid-filled chamber situated actually within the mesoderm.