In traditional two-kingdom systems, the multicellular animals were referred to broadly as metazoa to distinguish them from one-celled "animals", the protozoa. In our classification scheme, the traditional protozoa belong to Kingdom Protoctista; in our system, animals may be defined as multicellular, heterotrophic, diploid organisms that develop anisogamously—from two different haploid gametes, a large egg and a smaller sperm. The product of fertilization of the egg by the sperm is a diploid zygote that develops by a sequence of mitotic cell divisions. These mitoses result in first a solid ball of cells and then a hollow ball of cells called a blastula. The defining characteristic of all animals is their development from a blastula. In most animals, the blastula invaginates, folds inward, at a point to form a gastrula, a hollow sac having an opening at one end. Further growth and movement of cells produce a hollow digestive system called an enteron if it is open at only one end, and a gut or intestine if it has developed a second opening.
The details of further embryonic development differ widely from phylum to phylum but are fairly constant within each phylum. Such developmental details provide very important criteria for determining relationships between the phyla. In many phyla, developmental details are known for very few species; in some, not for any. In all cases they cannot be summarized in a few words. For this reason, concise and precise definitions of the phyla cannot always be given here; our descriptions are rather more informal.
Although multicellularity is found in all the kingdoms, it has developed most impressively in the animals—their cells are joined by complex junctions into tissues. Such elaborate joints—desmosomes, gap junctions, and septate junctions, for example—ensure and control communication and the flow of materials between cells. These junctions—and there are more kinds than those listed here—can be seen with an electron microscope. Indeed, the study of cells in tissues, and of tissues in organs, is a science in itself—histology.
Most animals have ingestive nutrition: they take food into their bodies and then either engulf particles or droplets of it into digestive cells by the process of phagocytosis ("cell eating") or pinocytosis ("cell drinking") or absorb food molecules through cell membranes. Although behavior of various kinds (attraction to light and avoidance of noxious chemicals, sensing of dissolved gases, and so forth) can be found in members of all five kingdoms, the animals have elaborated upon this theme too, far more than members of the other kingdoms. Mammalian behavior is perhaps the most complex.
The animals are the most diverse in form of all the kingdoms. The smallest are microscopic—smaller than many protoctists—and the largest today are whales, sea mammals in our own class (Mammalia) and phylum (Chordata). The members of most phyla are found in shallow waters. Truly land-dwelling forms are found only in two phyla, Arthropoda and Chordata. Several phyla contain species that live on land in the soil (for example, earthworms), but these require constant moisture and have not really freed themselves from an aqueous environment throughout their life cycle. In fact, most animal phyla are aquatic worms of one kind or another. Most species of animals are extinct—these form the subject of another science, paleontology.
Of all organisms on Earth, only the animals have succeeded in actively invading the atmosphere. Whereas one can find representatives of all five kingdoms that spend significant fractions of their life cycle in the atmosphere (for example, spores of bacteria, fungi, and plants), none in any kingdom spend their entire life cycle in the atmosphere, and only animals fly. Active locomotion of animals through the air has been independently achieved several but not many times, and only in two phyla: Arthropoda, class Insecta, and Chordata, classes Aves (birds), Mammalia (bats and Homo sapiens only), and Reptilia (several extinct flying dinosaurs).
For many years (and even today), biologists divided the animals— protozoans and metazoans together—into two large groups: the invertebrates, which are those without backbones, and the vertebrates, which are those with. In fact, all animals except the Craniata, a subphylum of Phylum Chordata, belong to the invertebrate group. This invertebrate/vertebrate dichotomy amply represents the skewed perspective we have as members of Phylum Chordata. Our pets, beasts of burden, sources of food, leather, and bone—that is, the animals closest to our size and best known to us—are members of our own phylum. We now realize that, from a less species-centered point of view, characteristics other than backbones are more basic and reflect much earlier evolutionary divergences.
The animal phyla are described here in approximate order of increasingly morphological complexity. Two phyla of animals, set apart as Subkingdom Parazoa, lack tissues organized into organs and have an indeterminate shape. These are the newly discovered Placozoa and the well-known sponges. The other 31 phyla, constituting Subkingdom Eumetazoa (true metazoans), have tissues organized into organs and organ systems.
There are two branches of the Eumetazoa. One consists of radially symmetrical organisms, the coelenterates and the comb jellies. These animals are planktonic and thus face a uniform environment on all sides; their radial symmetry is both internal and external. All the rest of the 29 phyla show bilateral symmetry, at least internally.
The bilaterally symmetrical phyla may be divided into three groups, or grades: those that lack a coelom, those that have a body cavity but lack a true coelom, and those that develop a true coelom. What is the coelom? The process of gastrulation leads to the development of three tissue layers in all animals more complex than the coelenterates and ctenophores. These tissue layers, called the endoderm, mesoderm, and ectoderm (listed from the inside out), are the masses of cells from which the organ systems of animals develop. In general, the intestine and other digestive organs develop from endoderm, the muscle and skeletal materials from mesoderm, and the nervous tissue and outer integument from the ectoderm. In the coelomates, the mesodermal tissues open to contain a space that widens and eventually forms a body cavity in which digestive and reproductive organs, among others, develop and are suspended. This true body cavity is called the coelom. A pseudocoelom is an internal space that does not develop from a space surrounded by mesoderm.
For animals of two phyla, priapulids and gastrotrichs, the nature of the body cavity is controversial. Only embryological studies, which have not been made, can determine the origin of the body cavity.
Two groups, called series, of coelomate animals are distinguished according to the fate of an early developmental feature called the blastopore. The invagination of the blastula, the hollow ball of cells into which the animal zygote develops, is the blastopore. This embryonic structure enlarges as cells divide, grow, and move over each other. In animals of series Protostoma, the blastopore eventually becomes the mouth of the adult. In series Deuterostoma, the blastopore becomes the anus, the rear end of the intestine; the mouth forms as a secondary opening at the end of the animal opposite from the anus. The five deuterostome phyla are thought to have common ancestors more recent than the ones they have with any protostome phyla. However, this divergence occurred at least 570 million years ago, judging from the presence of both protostomes and deuterostomes in the Cambrian fauna.
Virtually all biologists agree that animals evolved from protoctists. However, which protoctists, when, and in what sort of environments are questions that are still actively debated. E. D. Hanson has amassed a great deal of information on the protoctist-animal connection but admits the problem has not been solved for the Eumetazoa. The Parazoa, or at least the Porifera, are thought to have evolved from the choanoflagellates (i.e., the choanomastigotes). This is deduced from the details of fine structure of the cells. It is possible, in fact likely, that the other animal phyla, especially the eumetazoans, had different ancestors among the protoctists.
From: Margulis and Schwartz (1988).